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 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a flat site where the ground conditions are known, but since the inflation crisis and material increases this is now outdated.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. 

Tips for stretching your budget

  • 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.
  • Allow for professional fees – and planning contributions.
    Section 106 contributions in particular can be considerable. Check out Allan Corfield Architects’ insightful article on fees
  • Comparing costs and negotiating. A good negotiator will compare different prices from materials suppliers, professional consultants and trades people and haggle to get the best possible deal. On a big project this could save you £50,000-100,000.
  • Check quote expiry dates.
    Post Covid many suppliers will only guarantee quotes for materials and work for a short period of time due to market volatility.
  • Project manage yourself.
    Being your own project manager for the construction work could save you money, provided you know what you are doing – but don’t underestimate the time you will need to do this.
  • Get stuck in.
    You may be able to learn relevant building skills and do some of the physical construction work. Or enlist r skilled colleagues or friends or family to help you.
  • Plan properly.
    Don’t be in a hurry in the early stages – remember, time spent planning and working out every detail up front will pay dividends later on.
  • Check your spec.
    Avoid a high-end specification – you could spend £300 on a bathroom suite, or £30,000.
  • Rationalise your design.
    Opt for a simple design – a basic rectangular building or simple roof profile is cheaper to build than more complex designs. The method of construction has price implications – for example a basic timber framed homes can be about 10% cheaper than brick and block homes.
  • Safe storage.
    Secure, waterproof storage on site will enable you to buy cheaply in advance, and keep materials and tools safe for when they are needed.
  • Communicate!
    Any changes to the plans must be communicated with the relevant contractors. At the very least, stick a dated copy of the latest amended plans to an inside wall, so contractors know about the wall you’ve moved, or the extra sockets you realised you needed.

Budgeting tools

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.

Cost calculators & other help

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 2024? 

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.

*Sloping sites

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.

Useful links

There are lots of helpful online estimating services such as Estimators.com or Quanti-Quote and many more, but if using free versions use a range so you can compare. 

There are also 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 designed for consumers or trade professionals.

BCIS Price Books

HBXL Building Software

Easy Estimate


Fast Estimate

Price Guides Direct

Spon’s Price Books

Equally, there are many sites for sourcing contractors, such as TrustMark, but due your due diligence to check that any trades or professionals have experience on large scale projects. 

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.