Five Major Benefits of Approximate Quantity Design for Improved Efficiency and Accuracy

In infrastructure fields such as waterworks, sewerage, and general civil engineering, it is required to efficiently advance a wide range of tasks from design and estimation to construction management. Cost reduction and improved construction accuracy are also constant objectives. However, under conventional procurement methods, drawings developed at the design stage often do not match on-site conditions, leading to frequent design changes during construction, which increase costs and cause schedule delays due to rework. One approach gaining attention in recent years to address these issues is approximate quantity design. This method involves planning and estimating during the design stage using rough quantities and placing orders based on those quantities; final quantities are then determined by detailed design and surveying at the site. Trial implementations are currently progressing for water and sewerage projects, among others. The national and local governments are also promoting the adoption of this “approximate quantity design procurement method,” which is expected to contribute to streamlining design and estimation tasks and to the smooth execution of projects. This article first explains the definition and mechanism of approximate quantity design, then concretely introduces five benefits it brings.

What is approximate quantity design?

Approximate quantity design is a design method in which the quantities required for construction are calculated roughly before detailed design, and construction costs and procurement are based on those approximate quantities. For example, in a water pipeline installation project, the total approximate amount of piping materials can be estimated by multiplying a standard unit quantity—calculated from past project data—by the length (pipe length), and that becomes the initial design quantity. At this stage, creating detailed route drawings or material summaries for the piping is omitted, and after contracting, the contractor determines the detailed piping drawings and accurate quantities based on trial excavations and survey results at the site. In other words, planning is done in the initial stage using approximate values based on standard cross-sections on the drawings or unit quantities per unit length, and these are updated to the confirmed quantities that reflect site conditions during construction.

In short, the key difference from the conventional method is that detailed design drawings are not prepared before procurement; instead, bidding is based on approximate quantities and detailed design is finalized after contract award. The flow can be compared as follows:

• Conventional procurement method: The owner finalizes detailed design drawings and design quantities → construction cost is estimated and tendered based on those quantities → the contractor constructs according to the provided design documents

• Approximate quantity design method: The owner calculates the design price and tenders based on approximate quantities → after contracting, the contractor produces and finalizes detailed design drawings and quantities based on site surveys → construction and contract settlement proceed based on those finalized quantities

The aim of approximate quantity design is to reduce effort in creating detailed design drawings, thereby streamlining design and estimation work, enabling earlier procurement, and facilitating smoother construction. Another important feature is that the contractor participates in detailed design, allowing design and construction to proceed seamlessly. This approach is being actively adopted for linear infrastructure works such as water pipeline installation and sewer pipeline construction, and expansion into other fields like road improvement work is expected in the future. So what specific benefits can be obtained by adopting approximate quantity design? The five main advantages are explained below.

1. Cost reduction

Introducing approximate quantity design improves the efficiency of design and estimation processes and eliminates waste, resulting in significant cost reductions. For the owner, simplifying detailed drawings reduces the time and personnel costs required for design work. Initial estimation can also be carried out quickly based on approximate quantities, shortening the lead time to tender and contract and providing cost benefits from an overall project schedule reduction. For example, if detailed design documents are unnecessary, design outsourcing fees can be substantially compressed.

On the contractor side, cost-reduction effects can also be expected. Because the contractor can produce detailed construction drawings after contracting, it becomes possible to design and construct optimally according to actual site conditions, reducing oversupply or shortage of materials and rework. Even if design changes or additional works occur, contracts that anticipate flexible quantity adjustments from the outset can limit large cost increases associated with changes. Reducing waste during construction improves the overall cost performance of the project.

• Reduction of design costs: Omitting the creation of detailed design drawings and quantity summaries greatly saves the labor and time required for design and estimation

• Reduction of construction costs: Design adjustments tailored to site conditions reduce material waste and unnecessary work, enabling efficient construction

• Reduction of indirect costs through shorter schedules: Shorter project duration reduces site management and miscellaneous overheads, lowering indirect costs

2. Strengthened coordination between design and construction

In approximate quantity design, some of the design work previously performed by the owner (for example, detailed pipeline design) is taken on by the contractor. This allows the contractor’s know-how and ingenuity to be reflected from the design stage, significantly strengthening coordination between design and construction. When contractors carry out detailed design themselves, they can create plans that are easier to construct—i.e., with higher constructability—reducing mismatches and construction problems on site.

Also, progressing design and construction as a single process reduces communication loss and smooths information sharing. Under the traditional flow of owner → designer → contractor, changes or feedback on site conditions were relayed stepwise; with the approximate quantity design method, because the contractor is directly involved in design adjustments, these can be reflected immediately. This enables quick responses to design changes and faster decision-making during construction. Furthermore, contractor involvement from the planning stage makes it easier to propose VE (value engineering) ideas or adopt new construction methods, which can lead to higher-quality, lower-cost projects for owners. For contractors, engaging in design work contributes to skill development for their engineers and offers benefits in terms of corporate technical capacity building and talent development. A unified design-and-construction structure (similar to the design-build approach) also clarifies risk-sharing between owners and contractors, making responsibilities in the event of disputes clearer. Overall, lowering the barrier between design and construction so the team can work as one toward the project is a major strength of approximate quantity design.

3. Compatibility with ICT-enabled construction

Another attractive feature of approximate quantity design is its compatibility with ICT-enabled construction (information-based construction) that uses digital technologies. By digitally managing quantity data from the design stage, it becomes easier to incorporate ICT into various processes through to construction. For example, using CAD or estimating systems to calculate approximate quantities reduces human error and automatically updates quantity calculations when design changes occur. Also, the detailed design drawings produced by the contractor after contracting are prepared as digital data, making them directly usable for machine control and as-built management during construction. With the momentum of the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction*, on-site ICT adoption has accelerated in recent years; approximate quantity design has high affinity with such cutting-edge technologies and can serve as a foundation for construction DX (digital transformation).

For example, combining the following ICT technologies is expected to further enhance design and construction processes:

• Use of 3D models (CIM): Creating 3D models of structures and terrain from the design stage to streamline quantity calculation and construction simulation

• Drone photogrammetry and laser scanning: Rapidly measuring site topography from the air or on the ground to acquire high-precision point cloud data and improve the accuracy of approximate quantities

• Single-person surveying with RTK-GNSS: Using high-precision GNSS receivers built into smartphones, etc., to perform as-built surveying that previously required multiple people, quickly and by one person

• Use of machine guidance: Integrating digital design data with construction machinery for automated control that enables precise excavation and embankment, shortening construction time

Using these digital technologies can reduce the time required for as-built measurements that used to take a survey team several days—for example, drone aerial photography can complete it in hours, and GNSS surveying can produce results on the spot—allowing point cloud data or coordinates to be acquired within the day. In addition, data and drawings can be shared instantly via the cloud, making progress reporting and design-change consultations with owners smooth. Combining ICT-enabled construction with approximate quantity design enables smart project execution that leverages data consistently from surveying and investigation through design, construction, and inspection.

4. Clearer explanations to the owner

Approximate quantity design also has the effect of simplifying and clarifying explanations and procurement interactions with the owner (the project client). Conventional detailed design documents often include technical drawings and complex quantity tables that make it difficult for owners to grasp the overall picture. In contrast, design based on approximate quantities provides intuitive quantity bases—for example, “X pipes per Y meters of length”—making it easier for owners to understand the scope of work and cost breakdown. Approaching the procurement stage with minimal necessary drawings and quantity information enables quick, simple sharing with the owner about “what will be constructed where and how much,” speeding up consensus building. Simplified submission documents also reduce the effort needed to prepare and revise explanatory materials.

Moreover, once quantities are finalized after contracting, showing comparisons with the initial approximate quantities and explaining the reasons for differences with data allows clear explanations to the owner. If additional excavation is required on site, you can convincingly explain, based on measured as-built values, that “an additional X m³ is needed because the volume exceeded the estimate using the standard cross-section.” Combining digital survey data and photo records enables visual presentation of changes as well as oral or written explanations, increasing the owner’s sense of understanding and acceptance.

For example, using a tablet’s AR (augmented reality) function to overlay the design model at the site enables the owner to intuitively understand “where pipes will be buried” and “how the terrain will change after completion.” Spatial information that is hard to convey with drawings alone can be visualized through AR displays and 3D models, aiding explanations to owners and notifications to local residents. As a result, owner interactions become smoother and trust can be strengthened.

5. Improved on-site responsiveness

Approximate quantity design also enhances the ability to respond to unexpected situations and changes at the site. By not fixing detailed design excessively in advance, there is flexibility to deal with conditions discovered during construction. For instance, if the location of underground utilities is offset or unforeseen obstacles are found, the contractor can promptly revise designs and coordinate with the owner, preventing major work stoppages or rework. Because the contract method anticipates quantity adjustments and design changes from the start, small on-site changes can often be processed smoothly within contract change procedures, minimizing impacts on the schedule.

Also, advancing construction while measuring as-built conditions makes the operation directly linked to post-construction inspection and as-built management. By measuring and aggregating as-built quantities immediately after construction and comparing them with design values, progress can be monitored with quality assurance rather than leaving work “as-is.” Recent advances in ICT mean more tools are available that allow site staff themselves to quickly grasp as-built conditions—drone surveying, terrestrial laser scanning, and high-precision positioning with RTK-GNSS, for example. Combining these tools with approximate quantity design enables rapid, quantitative decision-making when minor adjustments or additional work are required during construction.

Concrete examples of improved on-site responsiveness:

• Even if unexpected buried objects or obstacles are found underground, design changes and construction method adjustments can be quickly considered on site

• Measured as-built values can be immediately reflected in contract quantities, smoothing procedures for additional orders or deduction settlements

• If additional work arises from design changes, prompt countermeasures can be implemented on site to minimize schedule delays

In this way, approximate quantity design strengthens resilience to various on-site irregularities and makes it easier to complete projects according to plan. Previously, when problems arose on site, confirming with designers or owners took time; in a unified design-and-construction arrangement, more decisions can be made on the spot, and faster decision-making enables safe and smooth construction management. This capability is also a major advantage in emergency situations such as disaster recovery work, where the ability to judge and respond quickly on site is critical.

Conclusion

As described above, approximate quantity design delivers significant effects both in cost terms (productivity improvement) and technical terms (accuracy improvement). By unifying design and construction and leveraging ICT, this approach can be expected to streamline operations and improve accuracy across water and sewerage works and general civil engineering projects. It is indeed one approach to promote productivity improvement and DX (digital transformation) in the construction industry.

To fully realize these benefits on site, it is key to carry out surveying and as-built management easily and accurately. One helpful tool for that purpose is LRTK’s “simple surveying” function. By combining a smartphone with a compact positioning device, this solution allows site personnel to perform necessary measurements quickly without specialized surveying equipment or multiple staff.

Specifically, it supports photo-based dimensional measurement, overlaying design data with AR, cloud synchronization of measurement data, monopod-style measurement using a dedicated pole, and even volume calculations for excavated soil—all with a single device. For example, you can instantly measure pipe elevation differences and clearances from a photo, synchronize drawings and as-built data on the cloud for team sharing, and measure and share results on the spot to move quickly to the next action.

Actively using such advanced ICT tools can make the merits of approximate quantity design more tangible. By digitally advancing on-site practices one step further, it becomes possible to achieve both efficiency improvements and higher accuracy control—goals that have been difficult to realize simultaneously. Such initiatives may well become the new standard for future site operations. We recommend considering their adoption.