Budgeting for your build project

Budgeting can make or break a project. The more accurate your estimates, and calculations, the more likely you will build your dream home without any crippling over spends.

Things to keep in mind: a contingency of 10–20% will act as a cushion against unexpected costs. Most people suggest a 10% contingency for a flat site where the ground conditions are known, and 20% for a sloping site or one where you’re not sure what may lie below. A very rough guide is that one degree of slope costs an extra £1,000, so a 45% degree slope may give you fantastic views, and easier drainage, but may cost an extra £45,000 in groundworks. The exact amount depends on a number of relevant factors, including ground stability, the water table, and whether the spoil has to be taken to landfill. Groundworks are normally the part of building a house which is the most difficult to cost accurately in advance.

Changing your mind about the layout or specification of your home as you go can dramatically increase the costs. So work out exactly what you want before you start on site, and then stick to it. The more time you spend planning in advance; the more likely you are to keep to your budget.

On a new house you will probably be eligible for a fair sized VAT refund – typically you may reclaim about £10,000 on the cost of the building materials you use. But you have to pay the VAT out before you can reclaim it, you have to keep good records, and you have to reclaim the VAT within three months of completion. About a third of all self builders fail to do this, so they miss out of a big tax refund.

Top Tips for stretching your Self-build budget

For general guidance on how to make your budget go as far as possible, see:


Self-build budget tools – how much does it cost to build

There are several free online ready reckoners that help you estimate the rough cost of your proposed home. It is well worth comparing several as costs may be calculated on previous year costings, so a cross section will provide a clearer picture. In particular, materials increases and shortages have had a significant impact on build costs, with many contractors unable to guarantee quotes more than a few weeks ahead.

ASBA, the Association of Self Build Architects has some useful resources, including a budgeting sheet that sets out some standard costs. Check out its resources here.

Sometimes a self builder will employ a quantity surveyor (QS) or a building estimator to prepare really detailed construction estimates for them. To find a good QS or estimator, ask other self builders near you who they used and if they would recommend them. You can also get details of local QSs from the Royal Institution of Chartered Surveyors.

If you are hunting for a building estimator, try the The Chartered Institute of Building using “estimating” as a search term.

You could also try:

There are also a growing number of online estimating services such as:

Finally, there are a number of paid for software packages to help self builders work out their own construction costs, and there are several building cost guidance books available. Check:

Please note, NaCSBA cannot endorse any of these tools or services, so research and compare before committing to one or another.