Detailed Guide to the Preliminary Design Process: Basic Steps and Practical Tips

What is preliminary design? Its purpose and importance

Preliminary design refers to the early-stage work in a construction project where the building’s scale and layout are planned roughly while simultaneously calculating an approximate construction cost. The main purpose is to check whether the proposal fits the budget before detailed drawings and specifications are finalized. Through preliminary design, you can derive an approximate construction cost from the building’s floor area and volume and quickly verify whether the project direction is reasonable. Cost accuracy at this stage is generally around ±20–30%, but gaining an early sense of cost enables planning with fewer reversals.

Preliminary design is important because it directly affects budget management and decision-making. If a budget overrun is identified early, plans can be revised or alternatives considered at an early stage. Conversely, if estimated costs are within budget, that provides a basis to move the project forward. This kind of cost visibility also reassures the client and helps build trust. It reduces the risk of proceeding with unrealistic plans that later require major redesigns or value engineering (VE), thereby improving the overall efficiency and success rate of the project.

In short, preliminary design is the process that answers the question, "Roughly how much will this project cost at this scale and scope?" By taking this step—the compass of a building project—you create a foundation to proceed confidently into schematic and detailed design. Obtaining a realistic budget perspective early leads to higher-quality design proposals and client satisfaction, making preliminary design an essential task for design professionals.

When preliminary design is used (change of use, land utilization, proposals, etc.)

Preliminary design is used in various scenarios. Here are the main cases.

• Feasibility study for change of use: In cases such as converting an existing office building into a hotel, change of use requires a rough design plan for adaptation and a preliminary estimate of renovation costs. Together with regulatory checks (seismic resistance, evacuation standards, etc.), preliminary design helps identify how much structural reinforcement and equipment upgrades might cost and whether the project is financially feasible.

• Effective use of idle land and development planning: For land utilization—for example, building on a parking lot or idle land—there is a need to know how large a building can be constructed and what the construction cost would roughly be. Preliminary design produces a layout and volume plan for the site, enabling consideration of potential gross floor area and number of floors. It also provides approximate construction costs as material to evaluate profitability (for example, whether rental income from an apartment project justifies the construction cost). In initial land-utilization studies, preliminary design is often the key to judging the viability of a business plan.

• Proposals and competitions: In public projects or large-scale projects where designers are selected via proposals, it is sometimes required to present an estimated construction cost at the proposal stage. While creative design is prioritized in proposals, the feasibility of realizing that design within budget is also evaluated. Therefore, it is necessary to perform preliminary design based on proposal drawings and renderings to calculate the expected construction cost of the submission. Proposals that deviate widely from the budget are unlikely to win, so balancing design and cost is essential. Preliminary design thus provides the supporting data that underpins a convincing proposal.

• Other cases: Preliminary design is also widely used in early project stages for internal evaluations of new businesses, government budget requests, assessments of the scale of renovations to existing buildings, and early tenant planning. Grasping a quick sense of "If it’s this size, it will cost about this much" early enables smoother Go/No Go decisions and scale adjustments.

As shown above, preliminary design serves as a decision-making tool in the early planning phase across many scenarios. Whether changing use or planning new construction, starting with simple planning and a preliminary estimate to grasp the overall picture is the first step toward a design process with fewer backtracks.

Basic steps of preliminary design (site survey, zoning, volume check, approximate construction cost calculation)

Preliminary design should proceed according to basic steps, not by guessing numbers. Below is a detailed explanation of typical procedures when conducting preliminary design.

• Site survey: Start by understanding site conditions. Confirm the site area and shape, elevation differences, and relationships with surrounding roads and other physical conditions. At the same time, investigate urban planning and building code regulations (zoning, fire protection designations, building coverage ratio and floor area ratio (FAR), height limits, setback regulations, etc.) to identify the maximum building size and constraints. As needed, check infrastructure availability (water, sewer, electricity, gas connections) and ground strength or ground-improvement history. For renovations or additions, include as-built surveys and structural investigations of existing buildings. The information gathered in the site survey forms the basis for zoning and scale studies, so it’s important to gather it thoroughly.

• Zoning (formulating the basic plan): Based on site conditions and the client’s requirements, perform zoning to determine how to place the building on the site and how to allocate internal spaces. In site layout planning, consider approaches, parking, views and sunlight when evaluating the building’s position and volume. As a rough interior plan (floor planning), estimate the required room layouts and area distribution for each floor. For example, “1st floor: common areas and parking; 2nd–3rd floors: office floors” or “in a hospital plan, place patient rooms on the south side with a central nurse station”—compile these performance-driven layouts as rough plans. Zoning verifies whether the desired gross floor area can be realized within site and regulatory constraints, so aligning site shape and building volume is critical. Setting the plan direction here makes subsequent volume checks and cost estimates more realistic.

• Volume check: Based on the zoning plan, confirm the building’s volume. Specifically, aggregate the area of each floor to calculate the gross floor area and check if it fits within regulatory limits (FAR) and meets the client’s required scale. At the same time, get a rough sense of building height, number of floors, and shape. If needed, create simple elevation or section drawings or a volume model (3D model) to verify proportions and compatibility with the surrounding environment. The purpose of the volume check is to determine whether the planned building’s quantity is appropriate for the site and requirements. For example, if the FAR is exceeded, reduce the number of floors; if there is room for expansion, plan for potential future additions. Once gross floor area and a rough building form are established, you can proceed to cost estimation.

• Calculation of approximate construction cost: Once the building’s volume (gross floor area, number of floors, assumed structural type) is decided, calculate the approximate construction cost. Several methods exist, but in early stages the unit-price per m² or per tsubo method is commonly used. From past similar project data, set a unit price such as "about X yen per m² for this use, scale, and structure" and multiply by gross floor area to derive a rough cost for the building core and shell. If the plan is more developed, you can also use a quantity × unit price approach for major items (e.g., foundation work ¥XX, structural work ¥XX…), summing by major categories. Regardless of method, it is crucial to check whether the calculated approximate construction cost is consistent with the initial budget or target cost. If there is a large discrepancy, you need to revise the scale or adjust specification grades. Summarize the approximate construction cost results as client or internal materials, and clearly state the cost assumptions and conditions at this stage (e.g., "estimate as of MM/YYYY; excludes exterior works and design fees"). By following these basic steps, you obtain an overall plan and a preliminary cost estimate as a basis for proceeding to schematic design.

The above outlines the broad procedure for preliminary design. In practice, these steps are repeated as needed to refine accuracy. For example, if the first estimate exceeds budget, revise the plan and re-do the area tally and cost estimate. Iterating between design and cost estimation helps find the optimal solution, and the process itself guides the project direction while aligning stakeholder expectations.

Tips for quantity takeoff and unit-price setting

When estimating construction costs during preliminary design, you need simplified quantity takeoff and unit-price setting methods distinct from detailed estimation. Below are some tips to achieve reasonable accuracy efficiently with limited information.

• Focus on key quantities: Concentrate first on the major quantities obtainable from drawings or rough plans. Gross floor area is almost always essential. In addition, you can roughly pick up exterior wall area (perimeter × floor height), areas of finishes (floor finish total area, interior wall and ceiling area), and structural frame quantities (approximate tonnage for steel frames, approximate concrete volume and rebar weight for RC). For parts not detailed on rough drawings (e.g., opening areas or fine millwork), estimate proportionally or include a buffer at this stage. The point is to ensure accurate quantities for items that significantly affect the total cost while lumping smaller elements into summarized estimates.

• Group items into large categories: For preliminary estimates, avoid the detailed breakdown of a bill of quantities and treat multiple construction elements as a single item. For example, calculate an "exterior wall finish" item that includes both substrate and finish materials, using exterior wall area × a one-line unit price. Similarly use items like "interior finishes (lump sum)" and "electrical systems (lump sum)" with approximate quantities and unit prices. Aggregating items prevents omissions and allows fast estimation without getting bogged down in details. Deciding the scope of each grouped item requires judgment and experience. Prepare templates from past preliminary breakdowns or government guidance documents to streamline the process.

• Set unit prices based on experience and data: Unit-price setting (¥/m² or ¥/m) is best informed by historical project data. Collect and use your own or peer companies’ contract amounts for similar projects to derive a ¥/m² unit price or use unit-price tables by trade. For example, having a market sense like "standard cost for a steel-frame office is about X yen per tsubo" is valuable. However, you must adjust unit prices for project-specific conditions: location (urban vs. rural, site constraints affecting crane usage), building complexity (complex plans are costlier), and finish grade (luxury finishes increase unit price). Also reflect market price fluctuations (material and labor cost rises). Refer to current construction price indices and manufacturer pricing so unit prices do not become outdated.

• Allow contingency for uncertain items: Many items are undecidable in early design—e.g., the need for ground improvement or pile foundations depends on further investigation. For preliminary estimates, assume provisional conditions (e.g., "assume standard ground: ¥XX") and either include a conservative allowance or list them separately as "contingency" or "risk items." For parts where new technologies might be used or future design changes are possible, apply surcharge unit prices or set aside contingency funds. Overly conservative estimates inflate budgets, but overly tight estimates can cause problems later. Including a reasonable cushion is key.

With these practices, you can produce a rough estimate in a short time during the preliminary design stage. Quantity takeoff and unit-price setting are skills that improve with experience; junior staff should study predecessors’ preliminary estimates to learn how figures were derived. Ultimately, the goal is to improve the intuitive accuracy of "this scale and use will cost about ¥XX," enabling designers to lead projects from both design and cost perspectives.

Flexible adjustment methods for changing design conditions

Architectural projects often undergo various changes during progress: client requirement changes, additional site or regulatory information, or external factors like sudden material price spikes. Being able to flexibly adjust plans in response to changing design conditions is an important skill. Considering change-management from the preliminary design stage improves your ability to handle major revisions smoothly later.

▼ Quick response to plan changes: For example, after establishing the basic plan the client may request "increase the building size by one step" or "change room usage allocations." In such cases, repeat the zoning → volume check → preliminary cost estimation process promptly. To allow flexible adjustments, design your first preliminary estimate in a reconfigurable format. Using Excel sheets or dedicated software where area and unit-price parameters can be updated to automatically recalculate costs saves rebuilding estimates from scratch. Preparing alternative plans A and B with their respective preliminary costs for comparison is effective (allowing presentation of cost trade-offs for each plan option).

▼ Handling uncertain conditions: Some design conditions remain undetermined at the preliminary stage—for example, "if the ground is weak, piling will be required" or "negotiations on FAR exemptions are ongoing." Prepare two cost scenarios: condition A costing ¥XX and condition B costing ¥YY. Simulating alternatives ensures readiness for either outcome. Also flag changeable items in the preliminary estimate as adjustable items so that when changes occur you only need to swap those portions.

▼ Sharing information with stakeholders: When conditions change, don’t keep it to the designer—share updates promptly with stakeholders and explain cost and planning impacts. For example, if a client requests a change that increases gross floor area by 10%, explain that the preliminary construction cost is expected to increase by about 10% as well. Providing quantitative impact helps clients make cost-aware decisions and avoids misunderstandings (e.g., surprise at a later cost increase). Similarly, share changes with structural and MEP specialists so they can reflect them in their estimates, maintaining overall design consistency. Real-time adjustments and team communication lead to robust project management.

▼ Use of digital tools: Employing digital design tools like BIM is effective for tracking changes. With a BIM model, updating the model automatically reaggregates area and quantity data after a design change. Recently, systems that link BIM models to preliminary estimate generation have emerged—allowing immediate cost re-estimation after a design revision. Such tools enable real-time, cost-aware design. Cloud services and affordable software for small and medium-sized firms make these capabilities increasingly accessible; proactively researching and adopting suitable tools is advisable.

By considering how to prepare for variability from the preliminary design stage, you can create flexible plans that withstand project uncertainties. Architecture is always dynamic, and the designer’s role in balancing cost and design under changing conditions is a key demonstration of skill.

Pitfalls to watch for in preliminary design and countermeasures

Because preliminary design is an approximate calculation, there are inherent risks and pitfalls. Here we summarize common traps and how to address them.

• Pitfall 1: Underestimating due to omissions: Limited information in the early phase can lead to overlooked items in the estimate—e.g., omitting exterior works, demolition costs, or furniture and fixtures, later causing "these items weren’t included" surprises. Such underestimation can lead to major budget overruns. Countermeasure: Use a checklist when preparing preliminary estimates to ensure major items are covered. Refer to templates used on past projects by experienced colleagues. Also add clarifying notes specifying what is excluded or provisional (e.g., "exterior works are not included" or "ground improvement included but subject to change after borehole investigation"). This transparency makes later explanations and adjustments easier.

• Pitfall 2: Overly optimistic unit prices: In a desire to fit the budget, you might set optimistic unit prices or quantities. If the detailed estimate later reveals higher costs, the difference becomes a significant overrun. Countermeasure: Base unit prices and quantities on evidence—for example, recent award prices for similar works. Being somewhat conservative at the preliminary stage is safer. For uncertain elements, include margins so that unexpected cost increases fall within ±30% error. If uncertain, consult specialists early (structural engineers, MEP contractors) to validate rough numbers. Use objective data and expert input to enhance reliability.

• Pitfall 3: Inconsistency with the design: Cost estimates that do not reflect actual design intent cause discrepancy—e.g., estimating on a low-cost specification while the design calls for high-end finishes, or failing to account for a layout that increases construction complexity. Such inconsistencies lead to later design changes and added costs. Countermeasure: Ensure the designer is involved in the estimate or that designers and estimators communicate closely. Share special design intentions so they are incorporated into the preliminary estimate. Conversely, estimators should give feedback on cost drivers (complex shapes, special methods) to prompt design revisions. Even when design and estimating are separate functions, cross-checks during preliminary stages secure alignment between design and cost.

• Pitfall 4: Preliminary estimates being treated as final: Preliminary estimates are provisional, but stakeholders may treat the figures as fixed and be disappointed when the amount changes after further design development. This typically stems from inadequate explanation that the numbers are estimates. Countermeasure: Clearly communicate the status of the preliminary estimate—for example, "This estimate is ¥XX with accuracy ±30%; it may change with detailed design." Document this in writing and update preliminary estimates at project milestones, sharing the trend so stakeholders see how estimates refine over time. This helps stakeholders regard preliminary numbers as forecasts and accept later adjustments.

By paying attention to these points and applying countermeasures, you can minimize risks in preliminary design. In essence, the keys are "comprehensiveness," "prudence," "coordination," and "accountability." Being aware of these pitfalls allows early detection and mitigation of potential problems.

Case example (how much can be inferred from a simple drawing and area data)

Let’s appreciate the power of preliminary design through a simple example demonstrating how much of a project’s profile can be seen from just a rough plan and area data.

Example: A client owns an approximately 500㎡ vacant lot near a city and wants to build a three-story office building with a gross floor area of about 900㎡. Provided information is a simple site diagram and the desired gross floor area.

First, the designer sketches a rough building placement plan from the site diagram. If the site area is 500㎡ and the building coverage ratio is 60%, the maximum floor area per story is 300㎡. With three floors, the desired gross floor area of 900㎡ is achievable—confirming it is legally feasible. If the FAR is 200%, the allowable gross floor area is 1,000㎡, so 900㎡ is within limits. These regulatory checks show that a three-story, 900㎡ plan is feasible on this site.

Next, zoning considerations proceed. Assume 1st floor for parking and entrance hall and 2nd–3rd floors as office floors. Draft a rough plan with approximate column and wall layout: 1st floor including parking about 300㎡, and 2nd and 3rd floors about 300㎡ each. Place elevator and stairs centrally, and allocate office and meeting spaces on each floor. Even from a very simple drawing, the building’s basic organization becomes clear.

Proceed to the volume check. With each floor about 300㎡ and an assumed floor-to-floor height of 3.5m (typical for offices), the total height is about 11m. This height is not excessive for the surroundings. Without a physical volume model, you can estimate proportions and solar impact from simple plans and section dimensions—for example, the site faces a road to the south, offering good daylight, and a three-story building likely won’t unduly impact adjacent properties.

With the plan validated, estimate the approximate construction cost. Assume a steel-frame office building with medium-grade finishes and equipment. From past similar projects, estimate a building unit price of around ¥250,000–300,000 per ㎡ for the building core and shell. For a 900㎡ building, ¥250,000/㎡ yields about ¥225 million and ¥300,000/㎡ yields about ¥270 million—so the building construction cost is roughly in the ¥200–300 million range. Adding ancillary works (exterior works and infrastructure connection) and design fees increases the total project cost, but this gives the client a ballpark figure to understand that the building construction cost is in the order of hundreds of millions of yen.

From these approximate figures you can also estimate a construction period. For a 3-story, 900㎡ office building, a typical construction period is about 8–10 months (e.g., foundation 1.5 months, structural work 3 months, finishes and equipment work 4 months plus contingencies). Actual durations vary by contract method and start timing, but you can give a client a schedule estimate like "If construction starts next spring, completion could be within the year." Thus, simply knowing building size allows you to produce a general schedule.

As shown by this example, even with a rough drawing and basic numbers, you can discern the project’s scale, cost, and schedule to a considerable extent. Clients can use this information to evaluate project profitability or approach lenders, and designers gain confidence in the project direction. Of course, these numbers are provisional and will change as detailed planning proceeds, but starting from realistic figures is far better than starting from nothing. Preliminary design quickly visualizes "Can it be built? How much will it cost?" and gives all stakeholders a tangible sense of the plan.

Use of digital tools and the speed of simplified surveying

Recently, various digital tools have been introduced into architectural preliminary design. By leveraging these tools, you can perform faster and more accurate early-stage studies than before.

▼ BIM and automatic estimating systems: As noted earlier, building a model with BIM automatically aggregates areas and volumes. If cost information is linked to components and finishes, the model can generate real-time preliminary construction estimates. Commercial 3D architectural and estimating software often include unit-price databases and rules so that inputting a plan can instantly output a preliminary estimate. Some major contractors have developed AI-based preliminary simulation systems that, given site shape, desired floor area and floor count, automatically propose building schemes and costs. Digital technologies enable rapid multi-scenario plan and cost comparisons. Cloud-based services and free preliminary tools accessible to small and medium offices make adoption easier, improving efficiency and proposal capability.

▼ Quick field surveying and immediate feedback: Digital benefits extend to site data acquisition. Historically, obtaining as-built site or measured building drawings took time and effort. Today, drone aerial photography, 3D scanning, and smartphone GNSS combined with simplified surveying tools enable rapid on-site data capture. A representative technology is LRTK. LRTK is a high-precision real-time positioning technology that enhances smartphones, allowing on-site acquisition of centimeter-level coordinates and elevation data simply by holding up a device. You can measure site boundaries, elevation differences, and the positions and heights of adjacent buildings on the spot and record them as digital terrain or point-cloud data. This allows designers to undertake immediate, accurate site measurements without waiting for surveying specialists, and to reflect the data in planning on the same day—significantly raising early-stage accuracy.

The speed of simplified surveying is remarkable. Tasks that once required weeks for official survey drawings can, with LRTK, enable same-day rough plan creation after a site visit. Import point-cloud data into CAD or BIM to begin zoning that accounts for site topography and surrounding buildings immediately. Digital tool adoption reduces the "site to office" time lag and improves proposal speed and client satisfaction.

Accurate early site information also prevents later surprises such as "a boundary isn’t where we expected" or missed elevation changes that force design revisions. In that sense, digital surveying tools also serve as risk mitigation. Smartphone-based, easy-to-use devices like those based on LRTK are affordable for small and medium design offices and contractor design departments and are practical to integrate into routine planning work.

Thus, digital technology is transforming preliminary design, combining rapid iteration with accurate site understanding to enable faster, more precise proposals. As new tools emerge, blending digital capabilities with designers’ experience will further elevate preliminary design quality.

Conclusion and natural induction to LRTK-based on-site surveying

This article has detailed the steps and key points of preliminary design. Confirming scale and obtaining a preliminary budget early in the project is a crucial process that serves as the project’s compass. By understanding the purpose and use cases, following the basic steps (site survey → zoning → volume check → approximate cost calculation), applying quantity and unit-price strategies for efficient estimating, preparing to handle changes flexibly, and paying attention to risks, you can significantly improve the accuracy and reliability of preliminary design.

For small to medium-sized design offices and contractor design departments, preliminary design is a core skill to be honed through daily practice. Combining experience, data analysis, and digital technologies enables rapid, persuasive planning and estimating. This not only helps win clients and projects but also gives designers a foundation to exercise creativity with cost awareness.

Finally, one promising technology to support modern preliminary design is LRTK-based on-site surveying. What used to take considerable time to capture site and building conditions can now be done quickly and accurately using LRTK-enabled smart devices. With LRTK (smart devices equipped with high-precision real-time positioning), designers can obtain required survey data on-site and incorporate it into plans the same day. This natural workflow leads to preliminary design grounded in real site conditions and higher-confidence proposals.

Preliminary design is not the finish line but the start of project success. As you begin, actively adopt modern tools and apply accumulated expertise. Aim for fast and reliable preliminary design tailored to your firm. Gaining a reliable compass at the project outset will make the project voyage far smoother—this is the promise of contemporary preliminary design.