Many homeowners first hear the term "structural calculations" when they apply for a building permit and the officer asks for more documents. This article explains what structural calculations are, which buildings the law requires them for, what goes into the report, who can certify it, and how it links to soil investigation, so you can prepare a complete set of documents the first time.
What Are Structural Calculations?
Structural calculations are a document showing, by engineering methods, that every part of a building's structure, from foundations, columns and beams to floors and roof, can carry its loads safely. They are prepared by a licensed civil engineer (COE license), signed with the engineer's registration number, and submitted together with the construction drawings in a permit application under the building control law.
Structural calculations serve two purposes. First, they are evidence to the local authority that the building was designed by a qualified professional and checked for safety. Second, they are the engineering record of the building itself: when the building is later extended, changes use or is inspected, future engineers can start straight from this report instead of back-calculating from scratch. For an overview of the whole structural design process, read A guide to RC structural design and civil engineering
Which Buildings Does the Law Require Structural Calculations For?
The simple rule: small houses are exempt, while larger buildings or those under controlled engineering work need structural calculations certified by an engineer. The commonly used criteria are summarized in this table.
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| Building type | Legal requirement |
|---|---|
| Residential house, floor area ≤ 150 m², max. 2 storeys, beam spans within the limit | Exempted: a site plan and outline drawings may be submitted, structural calculations not mandatory — except where built mainly of permanent and fire-resistant materials (e.g. masonry/concrete houses), which must include 1 set of structural calculations |
| Buildings over 150 m² | Must submit drawings with structural calculations signed and certified by an engineer |
| Buildings of 3 storeys or more · any storey 4 m high or more · column spans of 5 m or more · cantilevers of 2 m or more | Falls under controlled civil engineering work; a licensed engineer must design and certify |
| Public buildings, factories, large buildings | Structural calculations always required, often with additional requirements specific to the building type |
The controlled engineering criteria above are based on the Ministerial Regulation Prescribing Engineering Disciplines and Controlled Engineering Professions B.E. 2565 (2022), while the exemption criteria come from the building control law. An important caution: the Ministerial Regulation on Permit Applications under the Building Control Law B.E. 2564 (2021), Clause 2, states that public buildings, special buildings, or buildings constructed mainly of permanent and fire-resistant materials must always include 1 set of structural calculations with the application — an ordinary reinforced concrete house counts as permanent and fire-resistant construction, so it needs structural calculations even if it is 150 m² or less. (If the building is not controlled engineering work, the person preparing the calculations need not be an engineer; if it is, they must be a licensed engineer.) Local officers in each area also have discretion to request additional documents, so always check with the district office or Subdistrict Administrative Organization (SAO) before submitting. If the building falls under the soil investigation requirement of the Foundation Ministerial Regulation, see the summary at 2023 Foundation Ministerial Regulation summary
What Is Inside a Structural Calculation Report?
A standard structural calculation report has 5 main parts: design criteria, loads, material strength, member calculation results, and the certification page with the engineer's signature. They run from the overall picture to the details so that the reviewer can follow the design logic.
- Design Criteria: the reference standards, analysis method, structural system, and soil layer data from the soil investigation used to design the foundations
- Loads: dead load, live load according to the building's use, and wind or seismic loads where required by law — see which buildings must be designed for seismic loads in Earthquake-resistant building design under the 2021 Ministerial Regulation
- Material Strength: concrete compressive strength, reinforcing steel grade and other material strengths used in the calculations, which must match those specified in the construction drawings
- Member calculation results: checks of slabs, beams, columns and foundations member by member, showing they carry the forces safely within the criteria, with section sizes and reinforcement summarized to match the drawings
- Certification page and signature: the design engineer's name, license registration number (COE), signature, and a copy of the professional license attached at the end of the report
The point officers check most strictly is consistency between the report and the drawings — section sizes and reinforcement in the structural calculations must match the construction drawings for every member. If the drawings are revised later but the report is not updated, the documents contradict each other and can be rejected.
Who Can Certify Structural Calculations?
The certifying engineer must be a civil engineer holding a controlled engineering professional license (COE license) from the Council of Engineers Thailand. There are 3 main license grades commonly seen: Associate Engineer, Professional Engineer and Senior Professional Engineer (under the Engineers Act B.E. 2542 (1999) there is also a Special Associate Engineer grade, issued only for the work specified on the license). Each grade covers a different range of job sizes and types under the COE regulations. Small jobs such as ordinary houses can be signed by an Associate Engineer, while large buildings, high-rise buildings or complex structures require a Professional or Senior Professional Engineer according to the scope of the work.
Before hiring an engineer to design or certify, you can check the engineer's license status in the member lookup system on the Council of Engineers Thailand website (coe.or.th) by name or registration number. If the license has expired or its grade does not cover the job, the signed documents may not be accepted and you will have to start over.
The cost of hiring an engineer to design and prepare structural calculations depends on the size and complexity of the job. Read the detailed pricing method in How RC structural design fees are priced or see SPN's design services on the Civil engineering design page
What Documents Are Needed for a Building Permit Application?
The main document set for a building permit application consists of the application form Kh.1, drawings, site plan, specifications, structural calculations and the engineer's certificate. Structural calculations are an essential document for any building outside the exemption criteria.
- Permit application form (Kh.1) with the land title deed and a copy of the owner's ID card
- Drawings, site plan and specifications in the number of sets required by the local authority
- Structural calculations signed by the design engineer on every page or as required by the local authority
- Certificates from the design engineer and the supervising engineer with copies of their licenses (COE)
- Soil investigation results where the building falls under the Foundation Ministerial Regulation or the officer requests them
The number of sets and supporting documents vary by locality. For a complete checklist of the soil-related documents, read Soil investigation report documents for permit applications
How Are Structural Calculations Related to Soil Investigation?
One of the most important parts of the structural calculations is the foundation design, which must be based on the actual bearing capacity of the soil at the site from the soil investigation results. Without soil data, the engineer has to assume conservative values, so the foundations may end up more expensive than necessary; conversely, if the assumed values are better than reality, there is a risk of settlement and cracking later. Boring before design therefore makes the structural calculations accurate and prevents problems from the start.
SPN's soil investigation fee for residential houses starts at approximately THB 20,000–40,000 (2–3 boreholes, including basic lab tests and a report certified by an engineer). This price excludes travel and equipment mobilization, which are charged extra according to the distance to the site. See service details at Soil investigation and testing
This article summarizes criteria from the building control law and professional regulations for general understanding only. The criteria actually applied and the discretion of officers in each locality may differ. The design and certification of each building must be carried out directly by the engineer responsible for that project.