Extension Cost · St Albans

How much does an extension cost in St Albans?

St Albans prices at the South East baseline BuildLens builds from, in a cathedral city whose station terraces trade at London-adjacent values and expect finishes to match. A single-storey rear runs from a shell figure around £3,575 per m² to an all-in nearer £5,882.

Cost your St Albans extension See a sample report

The figure you find isn't the figure you pay

As everywhere, most cost guides quote a shell rate — the bare build. It is not what lands on your account. Calibrated to St Albans, a single-storey rear looks like this per square metre.

The headline
you find

£3,575

Shell only — structure, roof, envelope. Per m².

What you
actually pay

£5,882

All-in with a fitted kitchen, fees, contingency and VAT. Per m².

On a typical 30 m² rear in St Albans, that is roughly £150,000 to £175,000 all-in — against a shell headline nearer £107,000. The difference is the fit-out, the professional fees, the contingency and the VAT the shell figure never included. Where in that range you land depends heavily on the property.

Figures at the BuildLens South East baseline, priced on current trade day rates and live merchant pricing (2026). The all-in assumes an entry-level bespoke kitchen from local joinery; a higher kitchen tier moves it further.

What moves the number in St Albans

St Albans behaves like a London borough that happens to be in Hertfordshire — and its cost drivers follow suit.

A cathedral city priced like London

The Victorian terraces around the station trade at values most of outer London would recognise, and the finish expectations track the prices. The trades serving them price at the top of the South East band, with London twenty minutes away setting the alternative.

The conservation core is large

The city centre's conservation area is one of the larger ones in the region, wrapping the medieval core and the Georgian streets around it. Within it, external alterations face the usual controls; the terrace streets outside build more freely, and party wall matters are near-standard either way.

Clay and mature trees

Parts of St Albans sit on shrinkable clay with mature street trees close to rear gardens — a combination that drives foundation depths on some plots. It is a known local condition, priced after assessment rather than discovered mid-dig.

Where the money goes, itemised. The full anatomy of an extension — substructure through fit-out — and how BuildLens builds the number from method and live pricing, sits on the main extension cost page. This page is the St Albans calibration of it.

Extension cost — the full breakdown →

Common questions about extension cost in St Albans

How much does a single-storey rear extension cost in St Albans?

Calibrated to St Albans, the shell runs around £3,575 per m² and the all-in — with a fitted kitchen, fees, contingency and VAT — nearer £5,882. A typical 30 m² rear therefore lands roughly £150,000 to £175,000 all-in. The figure depends heavily on the specific property, which is what a desktop analysis of your address sets out.

Why do St Albans quotes differ from the national averages online?

Because most published figures are shell rates without fit-out, fees, contingency or VAT — and because they know nothing about St Albans: foundation depths driven by clay and mature trees on some streets, and the finish standard the station terraces expect. The quote is usually not high; the headline was incomplete for this property in this place.

Do trees and clay really change foundation costs in St Albans?

On the affected streets, yes. Shrinkable clay with mature trees nearby drives foundations deeper than the standard detail, and the difference is real money in the substructure line. It is a well-understood local condition — the point is to price it from an assessment of the actual plot before the budget is set, not to discover it when the digger is on site.

What pushes a St Albans extension over budget?

The property and the fit-out, usually — a kitchen or bathroom specified above the budget it was set against, and the local conditions above surfacing mid-job instead of being priced upfront. Both are predictable once the specific property is assessed, which is exactly what a BuildLens report does before you commit.