What Is a Retaining Wall, and Why Does It Need an Engineer?

A retaining wall is a structure built to resist the lateral earth pressure of a soil mass and keep the soil from sliding or collapsing. It is common wherever there is a change in ground level from filling, along riverbanks, in basements and on sloping sites.

What many people overlook is that a retaining wall is not just a "brick wall" but a structure carrying large forces at all times. Earth pressure increases with depth, so the total force on the wall grows with the square of its height (double the wall height and the total earth thrust rises about fourfold). It becomes even more severe when water builds up behind the wall (hydrostatic pressure) or when there is a surcharge on top, such as passing vehicles or a neighboring building.

The engineer must therefore check the wall's stability against several safety criteria: sliding, overturning and bearing capacity of the foundation soil, using the factors of safety in Clause 31 of the Ministerial Regulation on foundations B.E. 2566 (2023) for walls with a base (sliding ≥ 1.50, overturning ≥ 2.00, bearing ≥ 3.00) together with professional standards. That is why this work must go through a licensed engineer rather than being judged by eye or by a builder's experience alone.

Related: To understand how the different wall types compare, read Which Retaining Wall Should You Choose? 7 Types Compared, and for the safety criteria engineers use see Retaining Wall Factors of Safety Under Thai Law

How High Can a Retaining Wall Be Before an Engineer Must Sign Off?

The legal answer is 1.50 m. The Ministerial Regulation on Engineering Disciplines and Controlled Engineering Practice B.E. 2565 (2022), Clause 6 (20), classifies underground structures, temporary underground works and earth-retaining structures 1.50 m or more in height or depth as controlled civil engineering work. The designer and calculator must be a licensed civil engineer. But height is not the only factor; overall risk also matters. In general, walls fall into these groups:

  • Very low walls (below about 1 m) on level ground with no surcharge: low risk, but they should still be built properly, especially the drainage behind the wall
  • Walls 1.50 m or more in height or depth: controlled engineering work that must be designed and calculated by a licensed civil engineer. Above 2.50 m in height or depth, an Associate Engineer may not design it; a Professional Engineer or higher is required (see Scope of work for Associate Civil Engineers)
  • Walls close to a property line or neighboring building: should be designed by an engineer with signed calculations even below the threshold, because a failure of these walls affects both your own home and your neighbor's
  • Walls carrying a surcharge on top: e.g. a road, parking area or building foundation behind the wall. These always need engineering design, regardless of height

The 1.50 m threshold above concerns who must be the designer, which is separate from the threshold for applying for a building permit. Permit criteria are set by the Building Control Act and local ordinances, and the specific figures may differ from area to area. Before starting work, check with the relevant local authority or ask the design engineer to verify, to avoid problems with unpermitted construction later.

Comparison of a low-risk retaining wall with a wall that requires engineering design and sign-off, with four risk factors The left panel shows a low garden wall, no more than about 1 m high, on level ground with no surcharge on top: low risk, but it should still have weep holes and filter material behind it. The right panel shows a tall wall on a property line with a road or vehicles on top acting as surcharge, water accumulating behind the wall, and a neighboring house below close to the wall. The four risk factors are height, surcharge, water behind the wall, and proximity to the property line or neighboring buildings. Walls in this group must be designed and signed off by a licensed civil engineer, while the height threshold for a building permit depends on the local ordinance. The bottom strip summarizes the three key documents: soil data from investigation, construction drawings, and design calculations checking three stability cases. Which retaining walls need an engineer's design + sign-off Not to scale Low risk Low garden wall ≤ ~1 m Max. approx. 1 m high Level ground, no surcharge Still add weep holes + filter Natural soil ! Engineer design + sign-off required Tall wall, near boundary, surcharged Property line 1 High 2 Surcharge 3 Water behind wall 4 Neighbor house Near the wall * Permit height threshold → per local ordinance 1 Soil data Boring results (tall wall/soft soil) 2 Construction drawings Size, rebar, drainage system 3 Calculations Check FS sliding / overturn / bearing
The taller the wall, or with a surcharge on top or water building up behind it, the higher the risk. Walls in this group must always be designed and signed off by a licensed civil engineer. For the height threshold that requires a building permit, check the local ordinance for each area.
Caution: Do not confuse the controlled-engineering threshold (1.50 m, applied nationwide) with the building-permit threshold, which depends on the structure type, location and each local ordinance. Before starting work, check with the relevant local authority.

The 3D view below shows a common situation: a retaining wall on a property line with a neighboring house above. All four risk factors appear in one image: height, surcharge, water behind the wall, and the soil wedge behind the wall that may move if the wall shifts.

3D model of a retaining wall on a property line, with the factors that make engineering design and sign-off necessary Conceptual 3D view of a two-level soil block. On the left is high fill with a neighboring house close to the wall; on the right is lower ground. An L-shaped cantilever retaining wall stands on the property line. The front section shows the wall stem, the heel of the base under the fill, a gravel drainage layer behind the wall, the water level accumulated in the fill, and a light-red soil wedge behind the wall that may move if the wall shifts. A light-red band on the ground surface marks the zone of influence, which covers the front part of the neighboring house. The wall face has weep holes and a drain at the toe. The right panel is a quick checklist: height or depth of 1.50 m or more is controlled engineering work under the 2022 Ministerial Regulation; surcharge present; close to the property line or neighboring buildings; and soft soil or high groundwater. If any one applies, a licensed civil engineer should design and sign off the wall. 3D model: retaining wall at the boundary, why an engineer Not to scale 2 Surcharge House · cars · road behind wall 1 H ≥ 1.50 m Controlled eng. work (MR 2022) 3 Water behind Filter + weep holes 4 Soil wedge may move Influence zone widens with height Boundary Quick check: engineer needed? 1 Height/depth ≥ 1.50 m Controlled work, license req. 2 Surcharge on top House, cars, road, footings 3 Near boundary / neighbor Failure hits others' land 4 Soft soil or high GWL Less resistance, more water If any one applies → Licensed civil engr. designs and signs the calculations Building permit → Check the local ordinance Conceptual view, not to scale
3D model: ① a height or depth of 1.50 m or more is controlled engineering work; ② a house, vehicles or a road behind the wall act as surcharge; ③ water building up behind the wall must drain through a gravel filter layer and weep holes; ④ the red soil wedge is the soil that may move if the wall shifts; the taller the wall, the wider this zone, and it may extend under the neighboring house. The right panel is a quick checklist. Conceptual image (Image: SPN Soil Engineering)

What Documents Are Needed, and Who Signs Them?

When a retaining wall requires engineering design, the key documents are:

  • Construction drawings: showing dimensions, section sizes, reinforcement and drainage details behind the wall
  • Design calculations: showing stability checks for sliding, overturning and bearing capacity of the foundation soil, with factors of safety meeting the criteria (see The 3 FS checks under the 2023 Ministerial Regulation)
  • Soil data: for tall walls or soft-soil areas, a soil investigation should be done so that the bearing capacity and soil parameters used in the calculations are correct

The person entitled to sign the drawings and calculations is a civil engineer holding a professional engineering license from the Council of Engineers Thailand (COE), who signs with their license number. The license grade that may certify the work (Associate Engineer, Professional Engineer or Senior Professional Engineer) depends on the size and complexity of the work under COE regulations (read more: Which work each engineer license grade can sign). This document set is what accompanies the building-permit application to the relevant authority.

Note: SPN provides soil investigation and issues reports with design calculations certified by a licensed civil engineer, which can be used to support a building-permit application. Approval remains at the discretion of the relevant authority.

What Are the Risks of Building Without an Engineer?

Building a retaining wall without engineering design may look cheaper in the short term, but it carries several hidden risks:

  • Wall collapse or toppling: when earth or water pressure exceeds what the wall can resist, it may tilt, crack or collapse, causing injury, loss of life and property damage
  • Impact on neighbors: if the wall fails and soil flows onto adjacent land or damages a neighboring house, this can lead to disputes and legal liability
  • Legal and permit problems: building a structure that requires a permit without applying for one may lead to an order to modify or demolish it, and to problems when transferring or selling the property later
  • Higher repair costs: fixing a wall that has started to tilt or has failed usually costs far more than designing it correctly from the start

In short, the cost of design and calculations is small compared with the potential damage, so investing in proper design pays off in the long run.

How to Get Started, and How SPN Can Help

If you are planning to build a retaining wall, the recommended first steps are:

  1. Assess the risk: look at the height, the level difference, the soil conditions and adjacent structures
  2. Investigate the soil (if needed): for tall walls or soft-soil areas, a soil investigation gives the engineer real data for design instead of guesswork
  3. Have an engineer design and sign off: prepare drawings and calculations that meet the safety criteria, ready for the permit application
  4. Check permit requirements: consult the relevant local authority before starting construction

SPN Soil Engineering provides end-to-end geotechnical engineering services, including soil investigation and testing to obtain correct soil parameters, and geotechnical design, including retaining wall design, by a team of licensed engineers. We issue reports and calculations certified by a civil engineer that can be used to support a building-permit application.

For consultation or a quotation, call +66 64 746 9601 or LINE @spnth. Free quotation within 24 hours (fees depend on the scope of work and exclude travel and equipment mobilization costs, which are charged extra depending on the distance to site).

📌 Disclaimer: This article provides general information only and is not design advice for any specific project. The size, depth and details of a retaining wall must be designed and certified by the responsible engineer based on actual site data.