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 15-20% is essential to act as a cushion against unexpected costs. Traditionally, most people suggest a 10% contingency for a flat site where the ground conditions are known, but since the inflation crisis and material increases this is now outdated.
Groundworks are the most difficult to cost accurately in advance (see below for a note on sloping sites). 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.
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.
There are several free online ready reckoners that help you estimate the rough cost of your proposed home.
However, these can quickly become out of date or may refer to a single build method or presume that you are doing some work yourself, so cross check a range of tools.
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.
Estimating your self build costs is fundamental to your project planning, and costs can vary between £1,000-£2,500 m² realistically. It is possible to build for less but you will need to be doing a proportion of hte work yourself. If you plan to do this weigh up the difference between what you can earn and doing the work yourself but earning less as you will be on site.
Many companies serving the sector have, or link to, build cost calculators – but it’s worth finding out on what assumptions these make, and use these as a guide for further research only.
Self build specialist titles also have some great articles about costs and budgets, such as Homebuilding & Renovating’s How much does it cost to build a house in 2023?
Self build books, like the Housebuilder’s Bible by Mark Brinkley are also a great source of guidance.
Equally, you can ask what your contractor uses, as they may have a system that ties in with costs, such as Travis Perkins’ WholeHouse scheme.
ASBA, the Association of Self Build Architects has some useful resources, including a budgeting sheet that sets out some standard costs.
You may also employ a quantity surveyor (QS) or a building estimator to prepare detailed construction estimates.
Find a local QSs from Royal Institution of Chartered Surveyors (RICS).
Find a building estimator, try the The Chartered Institute of Building using “estimating” as a search term.
It can also be insightful to ask other self builders near you for recommendations.
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. However, you need to check if they are for consumers or trade professionals.
Note, NaCSBA and the Self Build Portal cannot endorse any of these tools or services, so research and compare before committing to one or another.
You should allow 20% for a sloping site or one where you’re not sure what may lie below.
Traditionally, 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.