How to Do Efficient Preliminary Design: A Practical Guide and Key Points for the Field

Introduction

To ensure the success of projects in the building and civil engineering fields, conducting a preliminary design in the early planning stages is indispensable. Preliminary design is the process of creating a rough design plan and estimating the required construction costs based on limited information before detailed drawings and specifications are available. By carrying out preliminary design at an early stage and checking the consistency between the design content and the budget, you can prevent major design changes or budget overruns that would require revising the plan later.

Preliminary design forms the basis of budget planning for clients and plays an important role for contractors (designers and builders) as part of cost control. For example, local governments use preliminary construction cost estimates based on preliminary design when requesting budgets, and private companies use them to assess project viability. For on-site designers and engineers, efficiently progressing preliminary design and quickly presenting accurate preliminary estimates helps projects get off to a smooth start. This article explains the basics of “what preliminary design is,” the workflow for progressing it efficiently, practical points useful on site, and operational considerations. Finally, it also touches on how to accelerate the initial stages of preliminary design by utilizing new technologies such as smartphone surveying.

What is preliminary design? Basic concepts and intended uses

Preliminary design is a simplified design and preliminary cost estimation performed in the early stages of a project. Specifically, based on the outline of the construction plan (purpose, scale, specifications, site conditions, etc.), you create rough sketches or models, extract the approximate quantities required, and calculate the construction costs. The objective is to grasp an approximate plan and cost before detailed design is completed and formal costing is conducted.

There are two major uses for preliminary design. One is for budget management and cost planning. Clients use preliminary construction cost estimates as the basis for securing project budgets, applying for financing, and requesting subsidies. For designers, appropriate preliminary estimates allow early correction of discrepancies between design content and target budgets, enabling an efficient basic plan without waste. The other use is to support project decision-making. Preliminary design makes it easier to judge “whether this plan is feasible” or “which alternative is most economical.” For example, when considering a building’s scale or structure, comparing preliminary costs makes it easier to choose a cost-effective option. Preliminary design is an important means of determining the project’s direction early and fostering a common understanding among stakeholders.

How to progress preliminary design efficiently: step-by-step explanation (Survey → Sketch → Quantities → Unit price → Estimation)

To progress preliminary design efficiently, it is important to divide the work into several steps and proceed in order. Below, the main flow of preliminary design is explained along five steps.

• Site survey and information gathering (Survey): Start by understanding the site and planning conditions. Investigate the site shape, elevation differences, ground conditions, locations of existing infrastructure (roads, water and sewer, electricity, etc.), sunlight and surrounding environment, and regulations (zoning, building coverage ratio and floor area ratio, building restrictions). Conduct hearings and collect related materials at this stage to gather as accurate information as possible that will serve as the design assumptions. The higher the survey accuracy, the better the accuracy of subsequent plans and cost estimates.

• Creation of an outline plan (Sketch): Next, based on the collected information, develop a rough design plan (basic concept). Draw rough floor plans and sections and determine the layout, scale, and structural form of facilities or structures. For buildings, sketch room layouts and number of floors; for civil works, outline routes and cross-sectional shapes. At this stage, emphasize the overall picture and feasibility rather than details, and if multiple options are possible, compare outline plans to determine the direction.

• Calculation of preliminary quantities (Quantities): Once the plan is set, calculate the main quantities based on that plan. For buildings, extract gross floor area and sizes/quantities of major components; for civil works, extract volumes of excavation and embankment, and lengths and areas of structures. You do not calculate quantities as finely as in detailed design, but it is important to grasp the approximate quantities for major trade works and elements. Using drawings from similar past projects or standard cross-section models improves accuracy. For uncertain items (e.g., equipment capacities or presence of special works), make assumptions and incorporate them into the estimate.

• Setting unit prices (Unit price): After quantities are determined, set the corresponding unit prices. A unit price is the cost per unit of each work or material, and multiplying it by quantity yields the amount. Use market unit prices, past project records, and public construction costing guides as references for unit price setting. For preliminary building estimates, it is common to use composite unit prices such as “XX work at △ ten-thousand yen per m²” or “XX equipment set at △△ ten-thousand yen.” Pay attention to the latest construction price indices and material price trends, and, if necessary, set unit prices that consider price fluctuations.

• Calculation of preliminary construction cost (Estimation): Finally, multiply the set quantities and unit prices to calculate the cost of each item, then sum them to estimate the project’s overall preliminary construction cost. Provide breakdowns for major categories such as primary building works, equipment works, and civil works, and include design fees, management fees, and contingencies as needed. Confirm whether the calculated preliminary construction cost fits the initial budget or target cost; if it significantly exceeds it, return to the plan and make adjustments (consider VE proposals, etc.). In this way, it is important to complete the alignment of cost and design content at the preliminary design stage.

By following these steps, you can proceed with preliminary design in an organized manner. Clarifying the flow from survey to estimation prevents omissions and streamlines the work.

Practical points for the field: survey accuracy, strategies for understanding site conditions, and tool usage

To perform preliminary design smoothly and reliably on site, there are several practical tips and strategies. Below are points practitioners should keep in mind.

• Improve survey accuracy: Conduct on-site surveys as thoroughly as possible to collect high-accuracy data. At the site, measure site dimensions and elevation differences, check ground investigation results and information on existing buried objects, etc. It is efficient to do desk research using drawings and geographic information systems (GIS) in advance and then perform measurements and confirmations on site. The more reliable the data obtained at the survey stage, the fewer rework iterations in later quantity calculations and planning, resulting in faster preliminary design.

• Strategies for understanding site conditions: Use photos, videos, and sketches in addition to text and drawings to gain a three-dimensional understanding of the site. Understanding the surrounding environment (adjacent buildings, road conditions, delivery routes, etc.) helps avoid overlooking design conditions. Having experienced technicians accompany the survey to share insights based on experience (for example, “this ground may require foundation reinforcement”) is also effective. Preparing a checklist helps collect information efficiently when survey time is limited.

• Use of IT tools: Utilizing the latest digital tools for on-site information gathering and preliminary design tasks can significantly improve efficiency. For example, obtaining terrain data of the site through drone aerial photography or simple laser scanning allows you to review details back at the office. Creating preliminary models in BIM or CAD software automatically calculates areas and volumes, reducing the effort of quantity calculations. Managing unit prices and quantities with estimating software or spreadsheets enables quick recalculation of amounts when changes occur. Recently, tablet and smartphone apps can also be used for drawing review and dimension measurement, facilitating information linkage between the field and the office. Actively adopt these tools to create an environment that enables accurate preliminary design in a short time.

Operational considerations: risks of overestimation and underestimation, and responding to design changes

There are several operational points to be aware of when performing preliminary design. Pay particular attention to the risks of overestimation and underestimation and to responding to subsequent design changes.

First, the risks of overestimation and underestimation. Even though it is a preliminary estimate, it is problematic if the estimated amount is far from the actual cost. Underestimating costs can later reveal budget overruns, jeopardize project continuation, or necessitate significant budget cuts. Conversely, overestimating costs can present an unnecessarily high budget at the planning stage, potentially leading to project rejection or forced scale-down. Preliminary estimates presented to clients are often treated as an “upper limit that must not be exceeded.” Therefore, it is important to present preliminary construction costs that are as realistic as possible. While the inclination may be to leave a comfortable margin on the safe side, excessive padding is discouraged. For uncertain factors, set an appropriate risk margin (contingency) while ensuring calculations are based on quantities and unit prices with clear justification. Also, clearly state the assumptions and specifications used in the calculation so that you can later explain the validity of the estimate.

Next, responding to design changes and coordination. Because the overall project is not finalized at the preliminary design stage, design content will inevitably change and be detailed as the project progresses into basic and detailed design. It is important to have a mechanism to promptly revise preliminary cost estimates when design changes occur. Update the preliminary estimate when moving to the basic design stage and evaluate the cost impact of major changes (such as increases or decreases in gross floor area or specification changes) as they arise. Share the assumptions and conditions used in the preliminary estimate with the client and project stakeholders, and report and consult on how those assumptions change when alterations occur. Even if design and estimating (costing) are handled by different persons, maintain close coordination to prevent information transmission loss. If there is an agreed budget frame established at the preliminary stage and cost increase factors arise during detailed design, consider adjustments through alternative proposals (so-called VE: Value Engineering). In short, since the plan and budget set during preliminary design serve as guidelines thereafter, continuous cost management is essential to maintain overall project consistency despite changes.

Conclusion

Preliminary design is a critical early step in a project, and how you approach it can significantly influence subsequent planning. Conducting preliminary design at the appropriate time and verifying the balance between budget and design content is the key to steadily advancing a plan. This article explained the basic concepts of preliminary design, how to progress it efficiently, practical on-site strategies, and points to be careful about. The main points are summarized below.

• Role of preliminary design: Calculate approximate design content and construction costs early in the project to form the basis for budget planning. Early preliminary estimates reduce the risk of budget overruns and enable control of design and cost.

• Efficient approach: Follow the steps “Survey → Plan creation → Quantity calculation → Unit price setting → Estimation” to carry out preliminary design comprehensively and smoothly. Using appropriate methods and data at each stage improves accuracy.

• Practical on-site strategies: Accurately grasp site conditions during the survey stage and use IT tools to gather necessary information quickly. Use BIM/CAD and estimating software to streamline quantity calculation and estimation, prepare to compare multiple options, and flexibly respond to changes.

• Points to note: Be careful of overestimation and underestimation and present realistic costs based on evidence. Share the assumptions used in preliminary estimates with stakeholders and immediately reflect cost impacts when design changes occur, maintaining alignment between design and budget.

By performing reliable preliminary design quickly, the entire project can proceed smoothly. While experience and intuition play a role, following a systematic procedure and leveraging modern tools can greatly improve the accuracy and speed of preliminary design.

Finally: Accelerating the initial stage of preliminary design with smartphone surveying using LRTK

Recently, a digital technology that supports productivity improvement on site has emerged: systems that combine smartphones with dedicated devices to perform high-accuracy surveying. A representative example is smartphone surveying using LRTK (a high-precision RTK-GNSS positioning system). By using LRTK, you can obtain accurate on-site surveying data in a short time with minimal equipment and personnel, dramatically speeding up the initial stages of preliminary design.

Traditionally, obtaining as-built survey drawings or terrain data at the planning stage required commissioning a professional surveying team and waiting for results. With LRTK, the responsible person can perform surveying on site using a smartphone. For example, by attaching a compact LRTK-compatible GNSS receiver to a smartphone and walking the site, you can acquire coordinates of site boundaries and terrain undulations in real time. The positioning accuracy is very high, with a horizontal error of about ±2 cm (±0.8 in), comparable to surveys conducted with conventional professional equipment. Measured data is saved and shared in the cloud instantly, so by the time you return to the office the whole team can view the latest site data.

By using such smartphone surveying in the preliminary design stage, the time required for on-site surveys and producing survey drawings can be drastically reduced. Because you can grasp ground elevations and site dimensions on the spot, you can immediately start creating outline plans and calculating quantities after the survey. LRTK also optionally provides simple 3D point cloud scanning and AR functions, allowing rough estimation of earthwork volumes from acquired point cloud data or overlaying planned facilities on the smartphone screen for on-site confirmation. These capabilities are particularly effective in the initial phase of preliminary design.

By incorporating smartphone surveying with LRTK, anyone can easily digitize an accurate “now” of the site and get a fast start on preliminary design. Because it improves both survey accuracy and time efficiency, it will be a reliable ally even when high-accuracy planning is required within a limited timeframe. Make good use of new technologies to carry out preliminary design reliably and quickly. Try adopting it in your next project and experience the benefits.