Key Considerations for Preliminary Design: 5 Points to Know to Avoid Failure

Introduction: Why is preliminary design prone to failure?

In civil infrastructure projects, the initial preliminary design is an indispensable step for determining project feasibility and securing budget approval.

However, if this early-stage design is inadequate, the risk of failures such as major cost overruns, construction schedule delays, and design changes increases. Because preliminary design is based on limited information, many uncertainties inevitably remain. If those uncertainties are underestimated or the necessary precautions are not taken and work proceeds, a series of "unforeseen" issues will occur in later phases.

In fact, construction sites often experience additional work and rework to the extent that people say, “it's rare for a project to be completed exactly as originally contracted.” The causes are varied—unforeseen ground conditions, disputes with neighbors, weather, and so on—but many stem from initial oversights or overly optimistic estimates. There are also frequent cases where misalignments in understanding among the client, designer, and contractor lead to situations of “this wasn't supposed to happen.” To avoid these failures, it is essential to firmly grasp the points to watch during the preliminary design stage. In this article, we specifically explain common failures in preliminary design and the five points to watch out for.

What Is Preliminary Design? Basic Definition and Purpose

First, let’s briefly clarify what “preliminary design” is. Preliminary design refers to the rough design and construction cost estimation carried out in the initial stage before entering detailed design (construction design). For an infrastructure project under planning, at a stage when exact drawings and specifications have not yet been finalized, approximate structural planning and quantity estimation are performed based on the information available, and a provisional construction cost is estimated. This becomes important documentation used for project feasibility assessment and budget requests (preliminary budget requests), or for considering procurement methods.

The purpose of preliminary design is, bluntly, to grasp early on "Is this plan feasible? Roughly how much will it cost?". In municipal public works, estimated construction costs are presented to secure annual budgets, and in private development rough estimates are required as material for investment decisions. Therefore, preliminary design demands reasonable accuracy in a short time, but because details remain undecided it is also characterized by a large margin of error. Generally, the accuracy of early-stage rough estimates is said to be about ±30–50%, and uncertainty becomes even greater for huge or complex projects. In other words, a rough estimate is merely a provisional guideline, and it should be recognized that it is not a definitive amount.

With the above in mind, preliminary design serves as a compass that determines the direction of a project, but depending on its accuracy and underlying assumptions it can also become a dangerous compass that steers you in the wrong direction. In the next section, we will take a closer look at five common failure patterns and key points to watch for in preliminary design.

Point 1: Misunderstanding Site Conditions and Insufficient Surveying

**Not accurately understanding the site conditions** is the most typical cause of failure in preliminary design. It is not uncommon for designers to rely solely on desk-based documents and old drawings when planning, and to proceed while misidentifying on-site conditions such as **topography, ground conditions, and existing structures**. For example, the following kinds of mistakes tend to occur:

• Misreading of topography and quantities: Calculating earthwork volumes using only old maps or rough elevation data can lead to large errors in actual cut and fill amounts. After construction starts, it may be discovered that “there’s not enough/excess soil,” resulting in major additional haulage or design changes.

• Overlooking ground strength and water table: Skipping pre-construction borings or geological surveys and using a standard foundation design when the site is actually soft ground. Or designing without accounting for a high groundwater level, necessitating additional ground improvement or drainage works after work has begun.

• Insufficient verification of buried objects and existing structures: Failing to identify old structures buried on site (old bridge abutments, box culverts, buried pipelines, etc.) in advance, with their existence only coming to light later. There are also cases where a high-pressure gas pipeline runs under the planned route, forcing an urgent redesign...

These are all problems caused by insufficient on-site investigation and surveying. In particular, in public works projects there is often no leeway to carry out detailed site inspections during the preliminary design stage, and conceptual estimates are sometimes developed based on a limited set of survey items. However, those consequences ultimately come back in the form of additional construction work and budget overruns.

Countermeasure: Be sure to “go see the site” and go collect data. As much as possible, the designer in charge should visit the site in person to confirm the topography and land use. Also, conducting simple surveys and trial excavations at an early stage will reduce the number of factors later claimed as “we weren’t told.” For example, in one condominium repair project, because the preliminary survey was inadequate, areas of exterior wall deterioration were discovered one after another after construction began, resulting in additional costs on the order of several million yen and extensions to the work schedule (caused by insufficient site investigation) [^1]. This is not something that only happens in building repairs; the same applies to civil infrastructure. Do not omit on-site verification just because it is the preliminary design stage, and be flexible enough to revise the plan based on the site-condition data obtained.

[^1]: An example of a repair project in which insufficient on-site inspection led to additional work: "If you sign a contract based only on a simple diagnosis without scaffolding, the risk of additional costs and schedule delays increases, so implementing a two-stage survey is indispensable." (From a column presenting a failure case by an exterior wall painting company in Saitama)

Point 2: Under- or Over-Design Due to Undetermined Design Requirements and Specifications

During the preliminary design phase, it is often the case that a project’s required performance and specifications have not yet been finalized. Even on the client side, while the “what” to be done may be decided, the “to what extent” (scale/level) may still be under consideration, or it may be subject to change due to consultations with relevant authorities. As a result, designers are forced to plan based on current assumptions, which carries the risk of under-design (designs that do not meet the required scale) or over-design (needlessly large-scale designs).

Under-design is exemplified by cases where the cross-section of a road is determined while the forecast of future traffic volume is still being examined, and it later turns out that the "traffic demand forecast was revised upward" and the design capacity is insufficient. If a road that should originally be four lanes was estimated as two lanes, major design changes become unavoidable. Conversely, an example of Over-design is when, erring on the side of safety and providing too much margin, unnecessary scale and functions are incorporated, driving up costs. For example, in estimating a sewage treatment plant, facility capacity might be calculated at the maximum projected scale due to uncertainty about future population, but if the demand forecast is later scrutinized, it may become clear that it was over‑speced. In this way, uncertainty in requirements is a double‑edged sword: misjudging it can lead to "built but insufficient" or "over‑engineered and over‑budget" outcomes.

Countermeasure: Make uncertain factors explicit and state assumptions. In the preliminary design estimate document, always record the assumptions—for example, "this estimate is based on condition ○○"—and agree in advance that it will be reviewed if conditions change later. It can also be useful to consider plans that allow a range. For example, by conducting comparative estimates of multiple options and indicating "this scale under condition A, this scale under condition B," you will be better able to respond flexibly to later requirement changes. In meetings with the client, share that "the specifications are provisional at this stage," and it is advisable to discuss the cost impact of scope changes in advance. In short, the key is not to proceed while leaving matters that are not yet confirmed ambiguous. Since the design is based on assumptions, all stakeholders need to understand that "if the assumptions change, the results will change."

Caution 3: Overreliance on Historical Unit Prices and Reference Examples

During the preliminary estimating stage, detailed quantity takeoffs cannot be performed, so estimates inevitably rely on past cases and standard unit prices. This is generally an effective approach, but you must not be overconfident in it. Relying too heavily on historical data risks producing incorrect preliminary estimates that do not suit the current project.

One common mistake is to blindly accept old unit prices or costs from similar projects. For example, if you used public construction unit prices from five years ago or in-house performance data as-is to make a rough estimate, recent spikes in material and labor costs could mean the actual cost has increased by more than 20%. In recent years, prices for steel and fuel have fluctuated greatly, and construction labor unit rates are updated annually. "It used to be ○○ yen/m², so it should be about the same this time" — applying such a simplistic assumption is extremely risky. You should also account for geographic and environmental differences. Applying urban construction costs to a rural site, or vice versa, can lead to large deviations because of differences in land costs, transportation expenses, and labor conditions.

Overreliance on reference examples is also a pitfall. For example, using the design of a previously successful similar project as a template can lead to overlooking the unique conditions of the current project. You might assume it's fine because the last site was flat, only to find this time the terrain is highly undulating and additional construction methods are required; or you may have reused a standard specification that, in practice, proved difficult to implement under the actual site conditions, and so on. Because each project has different conditions, it's important not to think “it's okay because there's a precedent,” and to always make adjustments and verifications according to the current conditions.

Countermeasure: Treat past data as a reference only, and adjust it with the latest information. It is true that accumulated past data can be helpful for preliminary estimates, but always check the latest unit price documents and market trends. For public works, verify unit prices using the latest “Estimation Standards” and materials from the “Construction Cost Survey Association,” and, if necessary, add an allowance for price increases. Also, when referring to similar past projects, it is essential to identify the differences between those projects and the current one. Compare from viewpoints such as how the scale differs, whether ground conditions are the same, and whether design standards or regulations have changed — and adjust for aspects that cannot be resolved by simple scaling. In short, 'use as a reference' but don't take it at face value. While basing your work on data, apply realistic judgment and aim for a highly reliable estimate.

Point 4: Overlooking Legal Regulations and Occupancy Restrictions

Infrastructure design is always subject to laws and regulations. If these are overlooked and you proceed with preliminary estimates, it may later become apparent during the permitting stage that “this design will not be approved” or “it does not meet the standards,” resulting in major rework. Additionally, if you do not identify restrictions on space use in advance—such as road occupancy or river occupancy—you may encounter situations where construction cannot be carried out as planned.

Overlooking regulations is typically manifested as non-compliance with design and safety standards. For example, in road design there are the Road Structure Ordinance and each municipality's road design guidelines; for bridges there are seismic design standards; for drainage facilities there are water quality and environmental standards—each prescribes specific threshold values and structural requirements that must be met. If you proceed from the preliminary estimate stage without checking these and assume "we can adjust during detailed design later," it may become apparent that the initial plan as-is cannot obtain approval due to insufficient width or excessive gradient. Restrictions under laws related to urban planning and land use (such as the City Planning Act, the Farmland Act, and the Forest Act) are also important. For example, if the planned site is in an urbanization control area, permission is required even for construction activity itself, and in zones for cultural property protection or areas regulated by the Natural Parks Act, construction methods and layout may be restricted. Ignoring these constraints when drawing up plans can force design changes at the permit-application stage.

Overlooking occupancy restrictions can also be troublesome. When using public property, prior approvals or consultations are required, such as road occupation permits under the Road Act or river occupation permits under the River Act. For example, installing structures on a road or burying pipelines that cross a road requires permission from the road administrator. If that is not taken into account during preliminary design and quantities are estimated on the basis of a 'plan to run a pipe here,' additional costs later may arise from permit conditions (such as night work or occupation fees), and in some cases route changes may be required. Also, crossings with railways and other infrastructure require caution. There are cases where a preliminary estimate for a sewer pipe passing under a railway turned out to be underestimated because discussions with the railway operator required double lining or special construction methods, resulting in higher costs than anticipated.

Measure: List relevant laws and permit requirements and verify them in advance. At the start of the design phase, identify the laws, ordinances, and consultation bodies that may be relevant to the project. It may span many areas—roads, rivers, water supply and sewerage, farmland, forests, urban planning, environmental assessments, etc.—but at the very least consult the responsible departments to check whether they apply. It is reassuring to include a note in the preliminary design document such as “This project may require permission under [X] law; depending on the conditions, changes may be necessary.” Furthermore, review the latest design standards and, in broad terms, confirm whether the design complies. For items that are likely to be designed right at the limit values (gradient, width, height restrictions, etc.), consider a slightly more relaxed alternative in advance to avoid problems later. Clearing legal and regulatory requirements is a fundamental prerequisite for advancing the project. “Ignorance is not an excuse,” so keep this in mind from the initial stages.

Point 5: Adjustment items and expected consultations with landowners are not reflected

Infrastructure construction, by its nature, requires extensive coordination with stakeholders. Depending on the project, there are various parties to consult: neighboring residents, landowners (rights holders), other infrastructure operators (electricity, gas, telecommunications, etc.), and even local communities and environmental organizations. A common failure in preliminary design is planning without taking these coordination matters into account.

One typical example is difficulty in land acquisition. If the planned site includes privately owned land, negotiations with landowners will naturally arise. At the preliminary estimate stage you may draw a route on paper and think "acquiring land around here will be fine," but when negotiations actually begin, unforeseen hurdles can emerge, such as escalating land acquisition costs, the provision of alternative land, and sometimes the need to address opposition movements. However, if such risks are not factored into the preliminary estimate and both land costs and the schedule have been estimated optimistically, the plan could later be at risk of stalling.

Also, coordination with existing public utility facilities is often overlooked. When road widening or sewer works require the relocation of utility poles and overhead lines, consultations and construction coordination with the relevant power and telecommunications companies are necessary. If the estimate ignores the costs and schedule for those coordination works, additional expenses such as “replacement piping works” and “temporary bypass facilities” may be incurred during the implementation phase, and the construction period can be extended. Furthermore, community commitments—such as local briefings and environmental protection measures—are also important. If agreements are added later, for example to install soundproof walls for noise and vibration mitigation or to restrict the hours construction vehicles may travel, they will affect the construction plan and costs.

Countermeasure: Identify discussion items with stakeholders and incorporate risks. List stakeholders who are likely to be involved in the project and consider what kinds of discussions or approvals will probably be required for each. For example, write down the expected items such as “advance explanation to the ○○ district neighborhood association is required,” “possible temporary use of part of land owned by △△ Corporation,” or “coordination with the ○○ Waterworks Bureau for water pipe relocation works.” On top of that, assess the impact if negotiations become protracted. Prepare alternatives (route changes or structural modifications), allocate contingency funds, and build schedule buffers. Of course, you cannot predict everything accurately at the preliminary estimate stage, but there is a big difference between “not considering it at all” and “having a reasonable expectation.” In particular, land negotiation tends to require both time and money based on experience, so you should coordinate with the specialized department early and develop a strategy. Local coordination is the key to project success. Make it a habit to consider negotiation plans with people and organizations together with the technical design.

Summary: How to Properly Utilize Preliminary Design

As discussed so far, preliminary design involves many uncertain elements and easily overlooked points. Misjudging site conditions, unclear requirement levels, misapplication of data, oversights in regulatory checks, and delays in stakeholder coordination——these are all traps that are easy to fall into, but conversely preventable if attention is paid in advance. If these points are firmly addressed at the preliminary design stage, you can reduce major course corrections in later phases and keep the project moving forward smoothly.

Finally, let's summarize the **key points for properly utilizing preliminary design**:

• Clarify assumptions: Always document the assumptions underlying the estimate (ground conditions, performance requirements, etc.) and share them with stakeholders. Agree up front that the estimate will be revisited if conditions change later.

• Incorporate risks and uncertainties: For unknowns and elements that may change in the future, provide an appropriate contingency (reserve). Consider optimistic and pessimistic ranges and estimate the impact in worst-case scenarios.

• Communicate with stakeholders: Don’t rely solely on designers; actively coordinate with the client and other departments. In particular, share and confirm the approach to coordination items and permits and approvals early.

• Continuous updates: An estimate is not a one-time deliverable; it should be updated as the project progresses. Whenever new information (investigation results or negotiation outcomes) becomes available, revise and refine the estimate so it always reflects the current situation.

The preliminary design is both the project’s compass and an adjustment valve. Its purpose is not to produce a perfect answer from the outset, but its mission is to provide a rational basis for decision-making while keeping future uncertainties in mind. For that reason, it is important to plan based on the precautions outlined here and on reliable information and sound judgment. An appropriate preliminary design will smooth the subsequent detailed design and construction and ultimately serve as a powerful guide toward project success.

Appendix: Getting Started with On-site Checks and Simple Surveys Using LRTK

To improve the accuracy of preliminary design and prevent failures, the key is how accurately you can gather on-site information in the early stages. That said, commissioning detailed surveys and investigations from external parties for every project takes both time and money. That is why tools that allow you to easily obtain on-site survey data are attracting attention. One such system is called LRTK（エルアールティーケー）. It is a cutting-edge tool that has recently become a topic of conversation among civil engineers, so some people may already be familiar with it.

What is LRTK? In a word, it is a groundbreaking device that transforms a smartphone into a high-precision surveying instrument. It is used by attaching a dedicated compact GPS receiver (antenna) to an iPhone or similar device, and by leveraging satellite positioning RTK (real-time kinematic) technology, it can improve position information— which would have errors of several meters with a standard smartphone GPS—to accuracy within a few centimeters. In addition, by integrating with the smartphone’s built-in LiDAR sensor and camera, it realizes an all-in-one set of practical field surveying functions such as 3D point cloud data measurement, on-the-spot volume calculations, and coordinate navigation to target points. In short, if you go to a site with this one device in your pocket, you can quickly perform on-site inspections and measurements that previously would have required assembling a surveying team and bringing equipment.

Benefits LRTK brings to on-site verification In the preliminary design stage, there are often needs such as 'I want to quickly measure the site' or 'I just want to know the height or distance of this point.' By using LRTK, for example, you can continuously measure elevation differences along a planned route in a short time, record the positions of property boundaries on the spot, and instantly measure the distance to a concerning obstacle with a laser, allowing you to obtain exactly the 'data you wanted' right there. The collected survey data can be overlaid on maps or drawings on a smartphone for verification, and can also be saved to the cloud and shared with your team. This makes it possible to continue assessments back at the office based on accurate current-condition data, reducing later discrepancies of the 'it turned out different from what we thought' variety.

Additionally, LRTK’s ease of use accessible to anyone is another major advantage. Traditional surveying instruments required specialized knowledge, but LRTK is designed so that you can launch a dedicated app and start positioning with a single tap, and with intuitive controls you can collect points or perform scans. Because it is easy to operate even for technicians with limited field experience, they can collect on-site data as an extension of their daily work. This will be useful not only for preliminary design but also in construction management and maintenance situations.

In summary, the most important thing to avoid failure in preliminary design is to "know the site." As a means of doing so, simple surveying with LRTK can be an extremely powerful ally. If you haven't tried it yet, why not give it a try on site? You'll likely be amazed, thinking, "Is it really possible to obtain such accurate site information this easily?!" Make good use of technological advances to help achieve highly accurate preliminary designs and smoother project execution. At your site as well, LRTK can become a strong ally for fail-safe design.