A ground investigation tells you what is under your site before you build on it: the type of soil, how strong it is, where the water sits, and whether there is anything harmful in the ground. It is how your engineer chooses the right foundations, and it is often what the council, Building Control or your warranty provider will ask to see.
When do you need a ground investigation?
You will usually need one if any of these apply:
- You are building new homes or new commercial buildings. Warranty providers such as NHBC and Premier Guarantee expect the ground to have been looked at before foundations are designed, and Building Control needs to be satisfied that the foundations suit the ground.
- Your planning permission has a land contamination condition. Many councils ask for a desk study first and, if that finds a risk, an intrusive investigation and a plan to put things right before work starts.
- The land has had an earlier use. Old factories, garages, farmyards, railway land, filled ground and former mining areas all raise the chance of contamination, soft ground or voids.
- The ground is known to be difficult. Shrinkable clay near trees, high groundwater, sloping sites, peat or made ground all change the type of foundation you need.
- You are going deeper or heavier than usual. Basements, piled foundations, larger extensions and buildings over two storeys need better information about the ground.
- A lender, buyer or insurer has asked for it.
You may not need a full investigation for a small, single-storey extension on undisturbed ground with no history of earlier use. Even then, Building Control may ask for trial pits to check the ground at foundation depth.
Phase 1 and Phase 2: what is the difference?
- Phase 1, desk study and site walkover: a look at the site’s history and setting without digging. Old maps, geology records, mining and flood information and records of earlier uses are reviewed, then the site is walked. The report says what risks are likely and whether a Phase 2 is needed. On many sites it is the first thing the council asks for.
- Phase 2, intrusive site investigation: digging and testing on site. Depending on the site this can include trial pits, boreholes, window samples, soakaway tests and gas monitoring, with soil and water samples tested in a laboratory. The report gives the ground conditions, foundation advice and, where needed, a contamination assessment.
- After Phase 2: if contamination is found, a remediation plan sets out how it will be dealt with, and a verification report afterwards shows it was done. Councils often ask for both before the land contamination condition can be discharged. Our guide to contaminated land reports explains each stage.
Send us the address and your drawings, and we will tell you whether you need a Phase 1, a Phase 2 or both, with a fixed quote. Our team looks after the job from start to finish.
What the report tells you
- What the ground is made of and how strong it is
- Groundwater levels
- The foundation type and depth your structural engineer should design for
- Whether the soil can take a soakaway
- Any contamination or ground gas, and what needs doing about it
- Things that affect building on site, such as soft spots, old foundations or obstructions
How long does it take?
A Phase 1 desk study is usually ready within one to two weeks of instruction. A Phase 2 investigation usually takes three to six weeks from the site work to the final report, depending on the laboratory testing and whether gas or groundwater monitoring is needed. We confirm the timescale for your site when we quote, so you can plan it into your programme before the foundations are designed.
What we need from you for a quote
- Site address and a site plan with the boundary marked
- What you plan to build (number of units, storeys, any basement)
- Your planning decision notice, if you have one, so we can see the conditions
- Anything you already know about the site’s past use
- Access details for the site
Related services: Geotechnical Site Investigation, Contamination Reports, Flood Risk Assessments. You may also need a utility survey before anyone digs.
Frequently asked questions
Do I need a ground investigation for an extension?
Not always. For a small single-storey extension, Building Control often checks the ground when the foundation trenches are dug. For larger extensions, sites near trees, clay soils or filled ground, an investigation first can save redesigning the foundations later.
Is a ground investigation the same as a site investigation?
Yes. The two names are used for the same work. “Geotechnical” refers to the strength and behaviour of the ground; “contamination” refers to anything harmful in it. Most investigations look at both.
Can I skip Phase 1 and go straight to Phase 2?
It is not usually a good idea. The desk study shows where to dig and what to test for, so the Phase 2 is better targeted and often cheaper. Councils normally expect to see the Phase 1 first.
Will the council ask for a ground investigation?
If your planning permission has a land contamination condition, yes. Read the conditions on your decision notice, or see our guide to discharging planning conditions.
Does my warranty provider need it?
Warranty providers want to see that the foundations suit the ground. On new builds they will usually ask for site investigation information, especially where hazards are suspected. See our guide to the 10 Year Structural Warranty.
Do you cover my area?
Yes. We arrange ground investigations across England and Wales, including Birmingham, Coventry, Leicester, Nottingham and London.
Last reviewed: October 2026. General guidance for England only. Every site is different, so get project-specific advice before relying on it.
More guides in the Developer Knowledge Centre · Services: Geotechnical Site Investigation, Contamination Report