Costs & Budgeting
Hidden costs to budget for on a building project
The costs that sit outside the headline build price, and catch people out when they're not budgeted for.
Beyond the build itself, most projects also carry design and drawing costs, the planning application fee if one's needed, a structural engineer's input, a party wall surveyor if a shared boundary is involved, Building Regulations fees, and a contingency for the unexpected. None of these are optional extras - they're part of the real cost of the project, and worth budgeting for from the start rather than discovering partway through.
Last updated 20 August 2026
Key points
- Design, drawings and statutory fees sit outside the build price, not inside it
- A structural engineer is usually needed, not optional, wherever a wall is altered
- A party wall surveyor's fee applies whenever a shared boundary is affected
- Contingency isn't pessimism - it's standard budgeting practice, especially on older properties
Design and drawings
Before a builder can quote accurately, someone needs to produce scale drawings - at minimum for planning or a Lawful Development Certificate application, and in more detail for Building Regulations and construction. This is a genuine cost that sits before the build price, not part of it.
Statutory fees
A householder planning application carries a government fee (see our planning permission cost guide for the current figure), and Building Regulations approval carries its own separate fee. Both are fixed, published costs rather than negotiable ones.
If a shared wall or boundary is involved, a party wall surveyor's fee is a further cost - more if each side appoints their own surveyor rather than agreeing to a single one.
Professional input
Most extensions, loft conversions and basement projects need a structural engineer's calculations, particularly wherever a wall is being removed or a steel beam introduced. This is a genuine, necessary cost, not an optional add-on.
Contingency
Setting aside a percentage of the budget for unforeseen items is standard practice, particularly on older properties, where what's uncovered once work starts (concealed damp, an unexpected drain run, a wall that isn't where the drawings assumed) isn't always predictable in advance.
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