Why do so many home builds spiral over budget? 

Even well-planned home extensions and renovations frequently end up costing thousands more than expected. As a general rule, it’s recommended you have a contingency budget of at least 10% of your project costs for any home renovation project. That’s potentially £5,000 of additional spending for the average single storey extension. Budgets can be blown by any number of factors, many aren’t controllable, but some are. Often, a misunderstanding hidden in the small print of quotes is a common culprit. Build Budget Expert, Owen Gartside looks at three areas where homeowners can get caught out:

1. What your builder didn’t include in the quote

The final agreed scope of work is the description of the work being done for the price quoted. It could be the scope of work for the whole building project, or just for a smaller amount of additional work, such as a kitchen fit. It is important for homeowners to know what is included in the scope, but it is just as important to know what is not included.

Items missing from the scope of work or being misunderstood can be a key reason for homeowners going over budget. Simply put, if it isn’t included in the scope then you haven’t been quoted for it, and if it needs doing then you will almost certainly have to pay for it.

Some examples of this include:

  • A family finishes their extension only to realise their garden has been destroyed — and landscaping was never included in the quote. The family then has to pay for landscaping work to restore the garden, despite not having any money set aside for this in the budget.
  • A homeowner doesn’t realise that the core quote from their builder is only for basic materials and that items such as internal doors, skirtings and light and socket fittings are not included. The homeowner then faces unexpected additional costs for all of these items.
  • A homeowner and the builder discuss fitting a bathroom. However, the written scope of work is very basic and there is a misunderstanding between the homeowner and builder as to what they believe is included. They may decide it is fair to share the liability for the mistake, but even 50% of the cost is still an unexpected extra expense for the homeowner.

2. The problems no one spots 

Unforeseen costs are why you should have a healthy contingency fund. Unforeseen costs refer to additional work that could not have been reasonably foreseen by anyone, whether homeowner, designer, or builder.

But there are things you can try and consider to help reduce the impact of any unforeseen costs. Think about each aspect of your project and ask questions such as: 

  • What is the ground like? 
  • How old is my house? 
  • Am I renovating areas of the existing house? 

Then you can look at what possible costs may be incurred, and you may be able to allow for these in your contingency funds or make savings by planning accordingly.

Some examples of this scenario, include: 

  • The builder has quoted for a standard foundation depth, but the building inspector has stipulated a deeper dig. The additional cost eats up a substantial chunk of the contingency budget.
  • The structural support in existing areas of the house is not sufficient. Additional steels will now need to be purchased and fitted.

3. How delays can quietly drain your budget 

Ideally, your builder will have given you a project timetable which will outline when key aspects of the project are likely to take place. This will help you plan various other aspects of the build, from purchasing items to moving out for a period, if necessary. However, projects can overrun, often due to bad weather, but there can be other reasons.

For example:

  • Missing a slot that another trade has reserved for you: This is where a busy kitchen fitter is booked in for a fixed date, but the floor isn’t ready, and the kitchen fit has to be rescheduled.
  • The family moves out for a planned short spell of building but some of the finishing items aren’t supplied in time which means the family has to spend longer in rental accommodation.

There are many ways to minimise the additional cost incurred by all the factors set out above. As an independent consultant, I advise homeowners that fully understanding their project is key if they wish to set a realistic project budget and stick to it.

During my consultations, we make a proper comparison and analysis of the wording of quotes so that the homeowner and builder both know exactly what is expected throughout the project. The homeowners are then receiving the right information to be able to make a decision and can have confidence that the quote they have received is what they should expect to pay.

Meet Owen Gartside, the Build Budget Expert, at the Homebuilding & Renovating Show, 27-28 June at Sandown Park, Surrey, and 18-20 September at Excel, London. Get your free tickets to the 2026 shows!

Featured image: Jacob Lund, Canva

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