Can I build on existing foundations?

Q: I’ve been looking at several plots for a self-build and one potential is being sold with existing foundations; the owner apparently started and then ran out of money. I’d ideally like to save some money by building on these foundations. However, what are the implications if I purchase this plot? Would I be able to build on them, or would it be better to get rid of them and start again? Would the new house still be classed as a self-build for VAT purposes? Also, how difficult would it be to change the floorplan at this stage?

- Ben Taylor, Via Email

Mark Brinkley answers: The existing foundations could be a money-saving bonus, but equally they might just be an expensive liability which you will have to dispose of. Your job in assessing this plot is to work out which.

You need to find out as much as you can about these existing foundations. When were they put in place? Who built them? And, critically, were they inspected by anybody in a position to approve them? This could be the local Building Control inspector, the warranty provider or a qualified structural engineer. If they were built under supervision – and it would be highly unusual for them not to be – then they should have been passed fit for whatever was to be built on top of them, and if you can find the relevant paperwork to prove this, then you should be able to build up off them with no problems.

If, however, you cannot find any record of the foundations having been inspected, you may be in trouble. Bear in mind that the building of foundations is sometimes carried out by developers to lock in a planning permission – once a start has been made, planning permission becomes permanent – and the work may have been carried out many years ago. It is possible that whatever records made at the time no longer exist. In this instance, no one is going to vouch for the adequacy of the work carried out and you may well find that you have to remove what has already been built, which will be very costly.

As for changing the floorplan, this rather depends on what you have in mind. If you want to move external walls, you would be required to go for a new planning permission (which might be a problem for other reasons, and which may or may not be successful). Adding internal walls should be less of a problem, but they may well require further foundation work on which you should seek professional advice. It would also be a good idea to carry out a survey of the existing foundations to check that they were accurately built. Finally, the existence of partly completed building work doesn’t make it an existing dwelling, so it will still be classified as a new build from a VAT standpoint.

Existing foundations: Friend or foe?

Existing foundations can be a cost-effective asset — many a contingency fund has been swallowed up by unforeseen problems stumbled upon when excavating for foundations. Yet where existing foundations do not meet Building Regulations approval, or are the old foundations of a demolished or damaged building and difficult to insure, it may be worth starting over.

Find more expert advice

Further reading:

• Replacement Dwellings
• A Guide to Groundworks and Foundations
• Foundations Explained