Key Takeaway
"How much will it cost?" is the first question almost every homeowner asks.
"How much will it cost?" is the first question almost every homeowner asks. The honest answer is that it depends on size, specification, the property and the site. This guide explains how extension costs are built up in St Albans and across Hertfordshire, and what early budgets often leave out. For a figure for your own project, use our free extension cost calculator and Project Feasibility Check.
Build cost per square metre
Builders usually price extensions per square metre. The Federation of Master Builders puts a fully finished single-storey extension at around £1,800 to £3,000 per square metre nationally in 2026, with London and the South East higher. St Albans and much of Hertfordshire sit in that higher band, because labour costs more and much of the housing stock is older.
As an illustration only: a 20m² single-storey rear extension budgeted at £2,500 to £3,500 per m² gives a build cost of roughly £50,000 to £70,000 before the extras below. Your builder's quote is what counts.
Single-storey, two-storey, loft or garage conversion?
- Single-storey extensions are the most common and usually the simplest route to a bigger kitchen or family room.
- Two-storey extensions often cost less per square metre, because the foundations and roof serve two floors. The total cost is still higher.
- Loft conversions add space without using the garden. See dormer vs Velux vs hip-to-gable loft conversions.
- Garage conversions are usually the lowest-cost option, because the structure already exists.
What pushes costs up in St Albans and Hertfordshire
- Conservation areas and period homes, where councils can expect matching materials. See our St Albans conservation area guide.
- Clay soils and nearby trees, which can mean deeper foundations.
- Restricted access, where materials have to be carried through the house.
- Large glazing, rooflights and high-end finishes.
The costs people forget
- design, planning and Building Regulations drawings
- structural calculations by an independent qualified structural engineer
- the council planning fee, where an application is needed
- Building Control fees
- Party Wall notices, and surveyor fees if a neighbour dissents
- a Build Over Agreement near a public sewer
- VAT on the build
- kitchen, bathroom and fit-out
- a contingency of 10 to 15%
Design fees: fixed fee or hourly?
Hourly design fees can climb as a project goes on. Hampshire Build works on fixed fees, with packages from £1,950 + VAT. See what each package includes.
Get a figure for your project
Our free Project Feasibility Check gives you an initial budget and planning view in a few minutes. Or book a free 30-minute consultation.
Hampshire Build is a residential architectural design practice based in St Albans, with more than 20 years' experience and a 95% planning approval rate.
FAQs
What is the cheapest way to extend a house?
A simple rectangular footprint with a straightforward roof. Where permitted development applies, it also avoids a full planning application.
Do I pay VAT on a house extension?
In most cases standard-rate VAT applies to extension building work. Check whether your builder's quote includes it.
Is a two-storey extension better value than single-storey?
Often per square metre, because the foundations and roof serve two floors. The overall project cost is higher.
How accurate is an online cost calculator?
It gives a realistic starting budget, not a quote. The final cost comes from your builder's price based on the approved drawings.