Why Do Houses Settle? 5 Common Causes
- Built on soft clay or new fill with an unsuitable foundation — soft clay keeps consolidating for years; if the foundation is shallow or the piles are too short, the house settles with the ground (Bangkok and its surrounding provinces see this problem most; read more in Bangkok soft clay and foundation work)
- Piles not reaching the stiff layer, or of unequal length — some piles bear on the stiff layer while others float in soft clay, causing differential settlement of the house
- Changes in groundwater level — e.g. groundwater pumping in the area causes additional soil consolidation
- Excavation or construction nearby — deep excavation or pile driving close to the house disturbs the soil beneath the existing foundation
- Soil around the house eroded or with voids — leaking underground pipes wash soil out until voids form under the floor
The symptom of "the ground around the house settling while the house itself does not" is normal for a piled house on soft clay — the ground settles naturally, but the house sits on piles that transfer the load to the stiff layer. What you really need to worry about is differential settlement of the house structure itself
Which Warning Signs Need an Engineer's Inspection?
The main signs to watch are diagonal cracks at door and window corners that keep widening, doors and windows that no longer close properly or sit askew, floors that tilt noticeably, a gap between an extension and the house that keeps opening, and water pipes under the floor that repeatedly break or leak for no known reason. Fine hairline cracks in plaster are usually not a structural problem — we cover crack types and their severity in detail in Foundation warning signs: which cracks need urgent repair. This article focuses on the next question: where does soil investigation come in?
House Extensions: Why Do They Often "Settle Away" from the House?
A classic problem in Thai houses is a rear kitchen or carport extension that settles and tears away from the house. The main cause is two foundation systems sitting on the same ground but settling by different amounts:
- The original house uses long piles reaching the stiff layer and barely settles
- The extension often uses short piles or spread footings on fill, which keep settling with the soft clay for years in soft-ground areas
The result is a tear at the joint, broken water pipes and a leaking roof. Soil investigation before extending answers the most important questions for this work: how deep is the stiff layer, and what type and length of pile should be used? — in the confined space beside a house, engineers often choose micropiles or jacked piles, as the equipment can reach narrow spaces, with the type and length chosen from actual soil data, not guesswork (compare pile types in Pile types: which one suits your project)
So When Should You Do a Soil Boring? By Situation
← Scroll to see the full table →
| Scenario | Is soil boring needed? |
|---|---|
| Light extension, e.g. a steel-frame canopy | Usually not needed, but consult an engineer about connections to the existing structure |
| Single-storey kitchen/carport extension on soft clay or fill | Recommended if there is no existing soil data for the house, to set the extension's pile length |
| Two-storey extension or a large added load | Should be done, as the load approaches that of a whole house (compare Does a 2-storey house need soil boring?) |
| House settled/cracked and needs foundation strengthening | Should be done, to find the cause and the depth of the stiff layer, and to design supplementary piles such as micropiles |
| Soil boring results for the original house already exist | Let an engineer decide; existing data may be used if site conditions have not changed |
A simple rule of thumb: the heavier the extension, the softer the soil and the less existing soil data you have, the greater the value of soil investigation, because one set of data serves to design the piles, the foundation and the structural joints (for why soil boring matters for every construction project, read Why soil investigation is needed before construction)
The Right Steps When a House Settles or Before Extending
- Have a structural engineer assess the site — look at the crack pattern, tilt and building history before concluding the cause
- Collect soil data — soil investigation to learn the soil profile, depth of the stiff layer and groundwater level; for a house, usually just 1 borehole at a suitable location is needed
- Design the fix/extension on actual data — e.g. specify the type and length of piles for the extension, or design underpinning piles for the foundation
- Separate the extension structure from the original house properly — provide a joint that allows differential settlement without tearing tiles, pipes or the roof, following the engineer's design (read the detailed principles of extension foundation design and structural separation in House extension foundations: how to design them so they don't settle or crack)
Summary: 1 Borehole Costs Far Less Than Fixing a Settled Extension
A settling house and an extension that settles away from it share the same root problem: "a foundation that does not match the actual soil layers." Soil investigation is the only way to know how deep the stiff layer is and how the soil settles. A soil investigation costs many times less than demolishing and fixing an extension that has already settled — for a 2–3 storey residence, SPN's soil investigation service starts at approximately THB 20,000–40,000 (for 2–3 boreholes, excluding travel and equipment mobilization, which are charged extra based on the distance to site), with a Boring Log report certified by a civil engineer (see pricing details and factors in Soil boring prices 2026)