House extensions, such as a rear kitchen, a storeroom or an enlarged ground floor, are the most common work homeowners undertake, but they are also the work that most often suffers from settlement and cracking, especially in soft clay areas such as Bangkok and its surrounding provinces. This article explains the geotechnical engineering principles behind the problem and how to design extension foundations that sit safely alongside the house.

Why Do House Extensions Often Settle and Crack?

The main cause is that the extension settles differently from the original house, producing differential settlement that pulls the two parts against each other until they crack. The original house usually already has piles down to the firm stratum, so it hardly settles, but a hastily built extension often rests on the ground or on shallow foundations without piles. On soft clay or fill, the extension gradually sinks over time.

When two parts that settle by different amounts are rigidly tied together, the forces from differential settlement show up as cracks along the joint, walls, floors and door and window frames. Common symptoms include the extension floor tilting away from the house, gaps opening at the corners of the house, water and drain pipes breaking due to settlement, and doors that no longer close properly. The softer the clay or the thicker the fill, the greater and longer-lasting the settlement.

Comparison of a house extension on shallow foundations that cracks from differential settlement with an extension that is structurally separated and piled down to the firm stratum The left image shows an original two-storey house on piles down to the firm stratum. Beside it is a single-storey extension on shallow foundations in soft clay or fill, bricked and plastered rigidly onto the original house. The extension sinks and tilts, and cracks form along the joint. The right image shows the same original house; the extension has its own columns, beams and foundations, with micropiles down to the firm stratum like the original house, and a movement joint sealed with a flexible material. Both parts therefore settle little and can move independently. The bottom strip summarizes three points: investigate the soil first, transfer the load to the firm stratum, and separate the structure with a movement joint. Extensions on soft clay: why they crack & how to separate Not to scale Risky extension Shallow footing on soft clay + rigid tie to house Separated extension Piles to firm layer + movement joint Soft clay / fill Stiff soil / dense sand Existing house · piles Sinks + tilts ! Cracks along joint Strained by diff. settlement Shallow foundation Existing house · piles Movement joint Flexible seal, no beam tie Micropiles to firm layer Own column–beam–footing 1 Soil boring first Soft-layer depth / firm layer 2 Load to firm stratum Piles / micropiles, not on fill 3 Separate structure Movement joint, no shared slab/roof
The original house, piled down to the firm stratum, hardly settles, while the extension on soft clay sinks, causing differential settlement and cracks along the joint (conceptual illustration, not a construction drawing)

What Is Structural Separation and Why Is It Needed?

Structural separation means designing the extension with its own foundations, columns and beams, so that it does not rest its load on, or tie rigidly to, the original house. Both parts can then settle and move independently. When the extension settles more, the original house is not pulled until it cracks.

Separation is needed because in practice it is almost impossible to make an extension settle exactly as much as the original house: the loads, the age of settlement and the foundation types are all different. Trying to "join them into one piece" usually has the opposite effect, shifting the forces to create cracks in places that cannot be controlled. Engineers therefore accept that differential settlement will occur and design the joint to accommodate that movement, for example:

  • Separate foundations and columns: the extension has its own piles/foundations, with no beams spanning from the original house
  • Leave a movement joint: make the joint between the two buildings free to move, and seal the surface with a flexible material instead of rigid brickwork and plaster
  • No shared floors/roofs across buildings: the extension's floor and roof carry their loads down through its own frame
💡 Structural separation does not mean they cannot touch

The two parts can still sit right next to each other and look like one house; structurally, they are simply not rigidly tied together. The key is a joint designed to move, not brickwork and plaster joining the two buildings into one piece, which is the most common cause of cracking.

What Does the Law Say About Different Foundation Types in the Same Building?

Clause 4 of the Ministerial Regulation Prescribing Building Foundations and Ground Supporting Buildings B.E. 2566 (2023) requires the foundations of a single building to be of the same type or kind, unless different types or kinds are necessary and the designer has considered differential settlement and found that it does not affect the building's structural stability. Building an extension on shallow foundations tied to an original house on piles, without an engineer considering this issue, is therefore risky both in engineering and legal terms (see the legal overview in 2023 Foundation Ministerial Regulation summary)

The 3D view below combines the key points of a structurally separated extension in a single image: soil data, piles, the extension frame, the joint, and pipes that cross the joint.

3D model of a structurally separated house extension with the key points that help reduce settlement and cracking Conceptual 3D view. A cut-away soil block shows the fill, soft clay and firm soil layers. The original house on the left stands on long piles down to the firm stratum. The extension on the right has its own columns, beams and foundations on micropiles down to the firm stratum, and a lean-to roof sloping away from the original house without resting on it. Between the walls of the two parts is a movement joint filled with an orange flexible material. Under the floor, pipes crossing the joint use flexible couplings. On the far right is a soil boring with SPT test points. The right panel summarizes Clause 4 of the 2023 Foundation Ministerial Regulation, that a single building must use foundations of the same type or kind unless necessary and the designer has considered differential settlement, and a five-item pre-extension checklist. 3D model: structurally separated extension Not to scale 4 Movement joint Flexible seal, no plaster over 3 Own columns–beams–footings Roof not on old house 2 Micropiles to firm layer Controls extension settlement 5 Pipes across joint Flexible couplings for diff. settlement 1 Bore first SPT at intervals Law + checklist 2023 Foundation Reg., Clause 4 A single building must use one foundation type unless needed, and the designer has checked that differential settlement won't affect the building's stability Check before extending Soil under extension Boring · SPT · water level Existing foundations Pile type/length (if dwg) Existing cracks/settlement Record before work Permit · setbacks Per Building Control Act Test installed piles As engineer specifies
3D model — ① Soil investigation and SPT testing before design ② The extension's micropiles reach the firm stratum to control settlement ③ The extension has its own columns, beams and foundations, and its roof does not rest on the original house ④ A movement joint sealed with a flexible material instead of plaster ⑤ Pipes crossing the joint use flexible couplings to accommodate differential settlement. Right panel: Clause 4 of the 2023 Foundation Ministerial Regulation and the pre-extension checklist. Conceptual illustration, not a construction drawing (Image: SPN Soil Engineering)

How Are Extension Foundations Designed?

The principle of extension foundation design is to make the extension settle as little as possible and as close as possible to the original house, by transferring the load to a soil layer with real bearing capacity rather than floating it on soft clay or fill. From an engineering standpoint, the steps are generally as follows:

  1. Know the soil first: carry out a soil investigation to determine the thickness of the soft clay/fill, the depth of the firm stratum and the groundwater level
  2. Estimate the extension's loads: consider the weight of the structure, roof, floors and the intended use to select the foundation type
  3. Select the foundation system: in soft clay, piles such as micropiles that transfer the load to the firm stratum are usually used instead of spread footings on the ground
  4. Design it separate from the original house: arrange the foundations, columns, beams and joints so that the extension works independently

Whether to use a spread footing or a pile foundation depends mainly on the soil conditions and the loads. On firm soil near the surface with modest loads, a spread footing may be enough, but on Bangkok's soft clay or fill, piles are usually needed to control settlement. For a detailed comparison of the two systems, read Spread footing vs pile foundation

⚠️ There is no ready-made formula for pile size/depth

The pile type, length and footing size must be calculated from the soil investigation results and the actual loads of each house. This article explains principles only, not design values. The design engineer should specify them based on the soil data and the building control law of each area.

Why Are Micropiles Suitable for Extensions?

A micropile is a small-diameter pile commonly used for extensions and foundation underpinning because it works well in confined spaces with low vibration, making it suitable for work right next to an occupied house. Its advantages for extensions include:

  • Access to tight spaces: small rigs/equipment can work behind a house or in a narrow soi (side street)
  • Low vibration: lower risk of affecting the original house and neighboring houses compared with driving large piles
  • Transfers load to the firm stratum: helps keep the extension's settlement close to that of the original house
  • Suits both underpinning and new work: can be used both for new extension foundations and for underpinning existing foundations that have settled

However, micropiles are not always the only answer. Their suitability, number and length depend on the soil type and loads. Compare them with other options in Pile types compared and have an engineer decide based on actual soil investigation results. Once installed, they should be load tested as specified by the engineer to confirm that the piles carry their design load (read House extension on micropiles: do you need a pile test?)

What Should Be Bored or Checked Before an Extension?

Before an extension, you should check the soil conditions beneath the extension area and the condition of the original house's foundations, so that the two parts can be designed to work together. The checks engineers usually consider include:

  • Soil investigation: the thickness of the soft clay/fill, the depth of the firm stratum, the bearing capacity and the groundwater level
  • Original foundation data: the type and depth of the existing piles (if drawings exist) to assess how much the original house will settle
  • Existing cracks/settlement: inspect and photograph existing cracks or settlement beforehand so that the extension work does not make them worse (see 5 signs of house foundation problems)
  • Legal requirements and setbacks: check setbacks, boundaries and permit requirements under the building control law (see Building permit process)

For typical residential extensions, a soil investigation helps you choose the right foundation from the start, which costs far less than repairing settlement and cracks later. For guidance specific to extensions, read Settling house extension: is a soil investigation needed?