The Hidden Costs of a House Extension (Beyond the Build)

When people budget for an extension they usually think of the build cost — but a realistic budget has to include the costs around it. Overlooking these is the most common reason a project runs over. Here are the additional costs to plan for. (For build-cost ranges, see our main guide, How Much Does a House Extension Cost in London?)

Professional and technical fees

• Design and drawings: concept design, planning or lawful-development drawings, and the technical package the builder prices and builds from.

• Structural engineering: calculations for steelwork and foundations — essential and not optional.

• Surveys and reports: measured surveys, and where needed soil, drainage or arboricultural reports.

Statutory and neighbour costs

• Planning or lawful-development fees: the council’s application fee, plus any pre-application advice.

• Building Regulations: building control fees and inspections, always required.

• Party Wall matters: if the Party Wall etc. Act 1996 applies, budget for surveyors’ fees and any awards.

On-site and practical costs

• Drainage and services: diverting pipes, moving manholes, and utility connections.

• Skips, parking permits and scaffolding: small individually, but they add up over a project.

• Fitting out: kitchens, bathrooms, flooring, decoration and furniture to finish the space.

• Temporary living costs: if you need to move out during the works.

And VAT

Most home extension work is standard-rated for VAT, so check whether quoted prices include it. Certain projects — for example some work to bring a long-empty property back into use — can qualify for a reduced rate, so it is worth asking.

Plan for the whole cost, not just the build

A good designer will help you build a complete budget from the outset so there are no surprises. To discuss your project, book a free consultation with Studio MASS: email info@studio-mass.com, call 0208 638 6727, or use the Talk to Us button above.

About the author

Robert McBride BSc(Hons) MFPWS is an architectural technologist and director of Studio MASS, a London practice specialising in residential extensions, conversions and new-build homes.