1. Why do I need to do a soil test?

You don’t, but it would be wise to. Soil types differ in their stability and need to be able to support the size and weight of your house. To begin with, Building Control can confirm the soil type in your area and may give you an indication of the best type of foundation to use. Digging trial holes will tell you more, and is particularly important for basement excavations, levelling a steep gradient, assessing contamination or if there are trees nearby. The roots will affect the moisture content of the soil and will determine the depth of the foundations.

2. What’s the difference between foundation types?

Trench fill foundations are filled with concrete to just below ground level. The sides and bottom of the trench are used to support the foundations. Often used in clay and chalk subsoil.

Strip foundations have a thinner base layer of concrete about 300mm thick. This spreads the load over a wide area so are often used for softer soils, like sand.

Raft foundations use steel reinforcing and concrete to cover the whole footprint and evenly distribute the weight of the building. Often used in soft clay, sand or peat soils.

Piled foundations are a more extreme foundation made from cylinders filled with concrete set deep into the ground. They’re used when the ground is very unstable.

3. How deep do I dig?

The depth of your foundations depends how far down you need to go to reach stable ground conditions and avoid frost penetration. On average this is around one metre.

4. How do I know which foundation system is right for my project?

Your engineer will make this decision, doing all the necessary calculations that take into account the ground conditions, the size of your house and the loadbearing required. Buildings Regulations Approved Document A contains guidance on structural stability: www.planningportal.co.uk/info/200135/approved_documents/62/part_a_-_structure

5. Can I change my mind about where I want to build on the plot?

If once you’ve cleared the site of vegetation and topsoil, you decide you want to move the footprint, then you’ll need to go back to planning with a revised drawing and request planning consent again. Otherwise your house has to be positioned exactly as per the approved plans.

6. Can I do the setting-out myself?

You can, but this is a critical stage and essential to get right and you may prefer the skills of a surveyor or your architect to do this for you. Agree a datum point, which is a fixed point that you measure all your heights from.

7. When do I dig trenches for the services?

At the same time as the foundations. The entry points need to safely pass through or above them to protect their load-bearing capacity. Trenches for drainage need to have the correct fall to the mains with access points. Buildings Regulations Approved Document H explains this is more detail:
https://www.planningportal.co.uk/info/200135/approved_documents/71/part_h_-_drainage_and_waste_disposal

8. What is the ‘oversite’?

Once the foundations have been formed, a compacted sub base of hardcore is laid over the top, then a damp proof membrane and poured concrete. Peg-out to mark the level of the concrete because this has to be spot-on ready for the frame. The oversite forms the internal floor of the house.

9. When does Building Control need to visit?

The building inspector will check and sign-off each of these key stages with the engineer:
– Prepare trenches before you pour concrete
– After the concrete is poured
– Preparation of the oversite
– Drainage before covered
– Drainage testing

10. Why do I need a contingency if I’ve got an engineered foundation drawing?

Ground conditions are unpredictable. Digging trial holes and doing detailed calculations could still mean changes to the foundation design if you come across unexpected tree roots, an underground spring, contamination, a sink hole, hidden sewer etc, once digging starts.