Why are soil conditions the biggest "hidden cost" of vacant land?

The price on the listing is only part of the cost — the other, invisible part is the foundation cost, which depends directly on the soil layers beneath the plot. Land with a shallow firm layer may be able to use spread footings or short piles, while land with thick soft soil needs long piles driven down to the firm stratum. Pile costs can therefore differ several-fold even for the very same building.

💰 Illustrative comparison (not actual prices — pile cost depends on type, size, length and each contractor):

  • Shallow firm layer → spread footing / short piles → foundation cost level 💰
  • Moderately soft soil → medium-length driven piles → level 💰💰
  • Very thick soft soil (e.g. the Central Plains lowlands) → piles 20 m or longer → level 💰💰💰

If you know this number before transfer, you can use it to negotiate the price, set your budget or choose another plot. If you find out after transfer, your only option is to pay more. (Read more on how soil boring prevents house settlement in Why soil investigation is needed before building a house.)

A tripod soil boring rig set up in the middle of an open grassy vacant plot, with crew in hard hats and reflective vests at work, plus a water tank and pump for wash boring
Screening boring on vacant land before the purchase decision — a small borehole, backfilled after the work, with no impact on the plot (Photo: actual SPN site)

The figure below compares two plots listed at the same price, on which the same house will be built. The whole difference lies below the ground surface, and a screening borehole is what reveals it before transfer.

Two plots at the same price, but with different foundation costs below ground Comparison of two vacant plots listed at the same price, on which the same house will be built. Plot A has a shallow firm layer and can use spread footings or short piles, so its foundation cost is low. Plot B is newly filled land over an old pond, underlain by thick soft clay, and needs long piles reaching the firm layer or sand layer, so its foundation cost is several times higher. Both plots have a screening borehole next to the house location. The bottom strip summarizes four things the boring results tell the buyer: the thickness of the soft layer, the depth of the firm layer, the SPT N-value of each layer, and the groundwater level. The figure is an illustrative comparison, not actual prices or pile lengths. 2 plots, same price — different cost underground Not to scale Plot A Same land price Fdn. cost ฿ Medium clay Shallow firm layer Bearing near surface Firm sand Screening hole 1–2 holes/plot Footing / short pile Plot B Same land price Fdn. cost ฿ ฿ ฿ New fill on old pond Filled-in pond Thick soft clay Lowland area Stiff clay Firm sand Long pile to firm layer 1 2 3 4 1 Soft layer thickness → pile length 2 Firm layer depth → pile tip level 3 SPT N-value → density per layer 4 GW level → excavation work Illustrative only — not actual prices or pile lengths
Two plots at the same price — Plot A has a shallow firm layer and can use spread footings or short piles, while Plot B is new fill over an old pond on thick soft clay and needs long piles to the firm layer. Items 1–4 are what the boring results tell the buyer (illustrative comparison, not actual prices or pile lengths)

What kind of land should have a soil boring before you decide to buy?

Every vacant plot bought for construction should have its soil conditions known before transfer, but plots showing the following signs should never skip this step:

  • Newly filled land — the fill has not yet consolidated and is often placed over weak original soil. From the surface there is no way to tell what lies beneath the fill: an old pond, former rice paddy or buried waste.
  • Former pond, pool or rice paddy, or close to a water body — areas that once held water often have thicker soft clay or silt layers than the surrounding area.
  • Water ponds for a long time after rain — a sign of slow-draining soil, usually clay or fine silt.
  • Neighboring buildings show settlement/cracking — a sign that the soil in that area has settlement problems.
  • Located in the Central Plains lowlands — Bangkok and its surrounding provinces have soft clay 12–18 meters thick (read about Bangkok soft clay). Knowing the actual soft-layer thickness on your plot helps assess the foundation system required.

The model below brings all 5 warning signs together on a single plot. Several can be seen from the surface, but the most important ones — such as an old pond beneath the fill or the thickness of the soft soil — can only be found by boring.

3D model of warning signs on vacant land that call for a soil investigation before the purchase decision 3D view of a vacant plot for sale, cut away to show the soil layers on the front and right faces. The surface is newly filled land with a leftover pile of fill soil and a puddle of water after rain. The front section shows an old pond filled in with fill soil and debris, underlain by thick soft clay typical of the Central Plains lowlands, stiff clay and a dense sand layer. On the neighboring plot to the left is a two-story house with cracked walls. On the land, a tripod drilling rig is carrying out one screening borehole, with a buyer and an engineer standing near the borehole. Numbers one to five mark the warning signs listed in the article. Depths are not to scale. 3D model — warning signs on land before buying Not to scale For sale 0 m 10 m 20 m Cracked neighbor Settlement sign 4 1–2 screening boreholes Know soil first New fill Unconsolidated 1 Ponding after rain Slow drainage 3 Old pond / paddy Filled-in pond 2 Thick soft clay Central lowland 12–18 m 5 Depth compressed · Conceptual illustration
3D model — warning signs on land before buying: ① newly filled land with leftover fill ② old pond or paddy filled over ③ puddles after rain ④ cracked neighboring building ⑤ thick soft clay typical of the Central Plains lowlands, plus the screening rig used to confirm conditions before transfer day (depths not to scale)

For land bought to build a factory or warehouse, there are additional issues such as floor load capacity and internal roads. See the dedicated checklist in Before buying land for a factory: an engineer's soil checklist.

A group of buyers and engineers visit the soil boring point where the SPN crew is working with a tripod drilling rig
Buyer's representatives visit the soil investigation on site before deciding to buy the land — a due diligence step that can be done before transfer day (Photo: actual SPN site)

Soil boring before buying land: how many boreholes and how long does it take?

For pre-purchase screening, 1–2 boreholes per plot are generally enough to establish the main soil profile, the thickness of the soft layer and the level of the firm layer — unlike boring for actual design, where the engineer sets the number of boreholes according to building size (see the principles in How many boreholes and how deep?).

Field work takes about 1–2 days per plot, and the complete report is delivered within about 7–14 working days after field work ends (see the detailed timeline in How long does a soil investigation take?), so it can be finished before the transfer date if you start early enough.

Soil investigation on someone else's land always requires the landowner's consent first. The approaches buyers use in practice are:

  • Ask the seller for consent directly — sellers who are confident in their land rarely object, because the borehole is small (about 10–15 cm) and is backfilled after the work.
  • Make it a condition in the sale-and-purchase agreement — have the due diligence period cover the soil investigation before the transfer date.
  • If the seller refuses any boring — the buyer should weigh this limitation in their own decision and price negotiation.

Contract conditions should be reviewed with a legal professional or your broker — SPN can provide engineering information but is not a legal advisor.

What do soil boring results tell a land buyer?

A screening soil investigation report answers 4 key questions for the buyer:

  • How thick is the soft layer — this governs how long the piles need to be.
  • How deep is the firm layer (sand / stiff soil) — the approximate pile tip level.
  • The density of each layer from the SPT N-value — a standard number every engineer reads the same way (how to interpret N-values).
  • Groundwater level — affects foundation excavation and basements.

This data lets the design engineer assess the foundation system from real data instead of the presumptive values allowed by law, which are safe but very conservative (see the comparison in No soil test: what values does the law allow?). The actual pile type, size and length must still be determined by the project's design engineer and cannot be concluded from the soil investigation report alone.

The soil boring crew performing an SPT with a tripod rig while engineers and stakeholders observe beside the borehole
SPT testing during boring — N-values counted on site at every depth are the key data in the buyer's report (Photo: actual SPN site)

Soil boring cost versus risk: is it worth it?

SPN's soil investigation service starts at about THB 20,000–40,000 (2–3 boreholes, including basic lab tests and a report certified by a civil engineer licensed by the Council of Engineers Thailand (COE), usable for a building permit application — see pricing details in House soil boring prices 2026).

This price excludes travel and equipment mobilization costs, which are charged extra depending on the distance to site.

Compared with land prices in the hundreds of thousands to millions of baht, and foundation costs that can differ several-fold between a good-soil plot and a soft-soil plot, the boring cost is a very small fraction of the value of the decision — and if you do buy the plot, the same report can be used straight away for foundation design, so you don't pay twice.

Summary: soil boring before buying land means buying "information" before paying the big sum

Vacant land bought for construction should have its soil profile known before transfer, especially newly filled land, land near water and land in soft-soil areas. Screening with 1–2 boreholes takes only 1–2 days of field work, with the complete report delivered within about 7–14 working days, giving you data for price negotiation, budgeting and the foundation design that follows. SPN's engineering team is ready to advise how many boreholes your plot needs and how deep — request a free quote within 24 hours.