Short answer: not required by law, but strongly recommended

From an engineering standpoint, soil investigation before building a 2-story house should always be done, especially in Bangkok and its vicinity and in provinces with soft clay such as Samut Prakan, Nonthaburi, Pathum Thani, Ayutthaya, and riverside areas.

Soil boring using the SPT method to collect soil samples and determine N-values for residential foundation design
Soil boring using the Standard Penetration Test (SPT) to collect soil samples and determine the N-value at each depth, so the engineer can design foundations and piles that suit the actual soil conditions of the plot (Photo: SPN Soil Engineering)
⚠️ Facts every homeowner should know

A 2-story house transfers its entire weight to the foundations and every pile. A foundation unsuited to the soil conditions is the main cause of house settlement, wall cracks and doors that no longer close properly over the long term.

What does the law say?

For typical residential houses (1–3 stories and not part of a large housing estate), the key points of the Ministerial Regulation on Building Foundations and Supporting Ground B.E. 2566 (2023) can be summarized as follows:

1. The law does "not require" soil investigation for a typical house

This regulation (Clause 5) does not require typical residential houses to submit a soil investigation report with the building permit application. It applies only to high-rise buildings, extra-large buildings, controlled-use buildings of 4 stories or more, and buildings of 4 stories or more in housing estates, which must have at least 3 investigation points.

2. But there is a "penalty-style restriction" if you do not bore

The same regulation (Clause 7) requires the design engineer to use very low soil bearing values instead, for maximum safety margin. For example, if soft soil is expected, the allowable bearing pressure may not exceed 20 kPa, and even for fresh hard rock it may not exceed 250 kPa — see the full table of presumptive bearing values for every soil type in Can you skip soil boring? Legal presumptive bearing values without a soil investigation

3. The consequences (why you should bore)

When the engineer is forced to use very low bearing values, the foundation must be over-designed — for example, using piles longer than needed or larger, thicker spread footings than usual — which drives up material costs and ends up many times more expensive than a soil investigation.

✅ Summary

The law does not require a typical house to have a soil investigation, but in practical and economic terms it is strongly recommended, so the engineer can design economically and safely for the actual soil and prevent long-term settlement. Many banks also ask for a soil boring report when considering high-value home loans.

In short, skipping soil boring is not illegal, but the designer must use low presumptive bearing values, whereas a soil investigation provides real data for a design that fits that particular plot.

Does a 2-story house need soil boring? Paths under the Ministerial Regulation on foundations B.E. 2566 (2023), with and without boring The flowchart starts from a typical 2-story residential house. Clause 5 of the Ministerial Regulation on Building Foundations and Supporting Ground B.E. 2566 (2023) does not require a soil investigation report; it applies only to high-rise buildings, extra-large buildings, controlled-use buildings of 4 stories or more, and buildings of 4 stories or more in housing estates. The path then splits in two. On the left, without boring, Clause 7 requires low presumptive soil bearing values, e.g. soft soil not exceeding 20 kPa, so the foundation must be larger or the piles longer, and construction costs many times more than boring. On the right, 2 to 3 boreholes about 20 to 30 m deep give actual bearing values from test results, allowing a right-sized foundation design, and the report can be used for permit applications and loan applications. At right is a plan of a 100 to 250 m² house plot with borehole locations, and the timeline: field work 1 to 2 days, laboratory 1 to 2 weeks, about 7 to 14 working days in total. 2-story house — bore or not: what differs? Not to scale Typical 2-story house Not high-rise / extra-large / ≥ 4 stories in estates Foundation Reg. B.E. 2566, Clause 5 No soil report required for a typical house No boring 1 Clause 7: presumptive values Low bearing values 2 Big footings / long piles Over-design to low values 3 Costs many × boring Real strata unknown 2–3 holes · 20–30 m 1 Actual bearing values From SPT + lab 2 Right-sized foundation Pile length to bearing layer 3 Usable for permits Accepted for bank loans e.g. soft soil ≤ 20 kPa Prevents uneven settlement Detached house 100–250 m² Example borehole locations House plan 2–3 holes at opposite corners Dashed = 3rd hole if soil varies How long? Field 1–2 days Laboratory 1–2 weeks Report Engineer-certified Total ~7–14 working days Allow ~1 month before build
Typical 2-story house under the foundation regulation B.E. 2566 (2023) — Clause 5 does not require a soil investigation report, but without boring, Clause 7 requires low presumptive bearing values, so the foundation must be generously oversized. Boring 2–3 holes about 20–30 m deep gives actual values for design and takes about 7–14 working days in total.

3 reasons a 2-story house should have soil boring, even when not required

1. Know the right pile length for "your plot"

The depth of the hard layer that supports pile tips varies from area to area — even within Bangkok, the Thonburi side is several meters deeper than inner Bangkok. Piles that are too short cause settlement; piles that are too long waste money. See soil layer depths and pile lengths by area across Thailand in Bangkok soft clay vs the provinces

2. Prevent uneven settlement (Differential Settlement)

Even if a house settles, you may not notice a problem if it settles evenly. Problems occur when it settles unevenly, causing:

  • Diagonal wall cracks that are hard to repair
  • Cracked floor tiles and debonded floor screed
  • Doors and windows that do not close properly
  • Broken water pipes under the floor and leaks

The model below shows a common case: the soft clay layer under the same house varies in thickness. When all piles are the same length with no soil investigation data, the side where pile tips are still in soft clay settles more.

3D model of a 2-story house with differential settlement where pile lengths do not match the actual soil layers 3D view of a 2-story house on a plot cut away to show the soil layers at the front. The soft clay layer thickens from left to right. All piles under the house are the same length, with no soil investigation data. Piles on the left reach stiff clay, while pile tips on the right are still in soft clay, so the right side of the house settles more: the house tilts, walls crack diagonally, doors and windows do not close properly, and the ground around the house subsides in a trough. Two dashed lines at the plan corners show where boreholes should be drilled before design, which would reveal the varying soft clay thickness before pile lengths are set. Settlement is exaggerated and depth is compressed for clarity. 3D model — uneven house settlement Not to scale Diag. wall cracks Doors/windows stick Right settles more Blue dash = old level Pile tip in stiff clay Pile tip still in soft clay Soft clay thicker here All piles same length Designed without boring Bore at plan corners first Reveals uneven soft clay Settlement exaggerated · depth compressed
3D model — a 2-story house with differential settlement. Piles on the left reach stiff clay, while pile tips on the right are still in thicker soft clay, so the house tilts and walls crack diagonally. Boring at the plan corners before design would reveal this difference (settlement exaggerated for clarity).

3. Save on long-term repair costs

Soil investigation for a residential house costs THB 20,000–40,000, compared with:

  • Wall crack repairs: 💰💰 often many times the cost of a soil investigation, and may need repeated repairs
  • Foundation strengthening (Underpinning): 💰💰💰 can be tens of times the cost of a soil investigation
  • Opportunity cost during repairs: incalculable

Note: the 💰 levels are a relative comparison; actual costs depend on the damage, the site and each contractor.

Cases where soil boring "may" not be necessary

There are some cases where it may be skipped, but this must be carefully assessed by an engineer:

  • A soil boring report from a neighboring project (within 50 m) carried out in the last 3–5 years
  • Hillside or hard ground with rock near the surface (e.g. many areas in northern and northeastern Thailand)
  • A small single-story house (under 100 m²) on clearly hard ground
💡 Engineer's advice

Even in the cases above, having clear soil data allows a "right-sized" foundation design. Sometimes the savings from not driving piles longer than necessary more than pay for the soil boring.

Soil boring for a 2-story house: how many holes and how deep?

← Scroll to see the full table →

House sizeNo. of holesDepthApprox. price
Detached house 100–250 m²2–3 holes20–30 mFrom THB 20,000–40,000 (whole job, incl. basic lab tests + engineer-certified report)
Large/luxury house 250+ m²3–4+ holes25–30+ mContact us for a free quote within 24 hrs

* Prices depend on location and job difficulty and exclude travel and equipment mobilization, which are charged extra according to distance to the site — contact SPN for an accurate quotation.

* Depths follow the EIT 2008 guidelines — boring continues to hard soil/dense sand per the engineer's criteria, with a recommended minimum of about 3 holes where soil layers are non-uniform.

Want price ranges for other building types, such as townhouses, shophouses or factories? Read House soil boring prices 2026 — budgeting guide, and see the engineering criteria for setting the number and depth of boreholes in How many boreholes and how deep?

How long does house soil boring take?

  • Days 1–2: the team mobilizes to site, sets up equipment, bores and collects soil samples
  • Weeks 1–2: laboratory testing for Water Content, Atterberg's Limits and Unconfined Compression
  • Final stage: the engineer analyzes the results and prepares the Boring Log and foundation design recommendations

The total is about 7–14 working days; if you plan about 1 month before construction starts, it will not affect your schedule. To learn what information the Boring Log contains and how to read it, see How to read a Boring Log, made simple

Who should do the boring? What to check before hiring

Most important, the report must be certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number, so it can be used for design and for building permit applications to the relevant authorities. You should also check that the company tests to ASTM standards, has its own laboratory and has a verifiable track record — see the full 7-point checklist in How to choose a soil boring company, and see the scope of SPN's soil investigation services on the Soil investigation and testing page.

Summary: bore before building a 2-story house

Although the law does not require every 2-story house to have a soil boring report, in terms of safety and long-term value, investing THB 20,000–40,000 in soil boring for a home you will live in for 30–50 years is very worthwhile — and to understand everything a soil investigation gives you, read Why do soil investigation before building a house?