Extension Cost Per Square Metre, Explained Properly

Why a rate exists at all

Builders use a rate per square metre because it lets everyone talk in the same units early on, before drawings exist. It is a planning tool, not a contract. Once a structural engineer has designed the foundations and the steel, the real number is built from measured quantities: cubic metres of concrete, linear metres of blockwork, square metres of roof covering, and the labour hours attached to each.

The consequence is that the rate can only ever be an average of jobs a builder has recently completed. If your site is unusual in any direction, the average is the wrong starting point and the survey will say so.

How the work type changes the rate

Different work types use an existing shell to different degrees. The table explains why a single rate cannot be transferred safely from one project to another.

How different project types build up a quote. Type of work | Main rate factors | What sits behind them. Single storey, basic specification — Ground, access, structure and openings — Simple rectangle, standard foundations, modest glazing; Single storey, higher specification — Glazing, joinery, services and finishes — Large sliders, rooflights and specified finishes; Two storey extension — Upper structure, connections and planning — Roof and foundation serve two floors; Loft conversion — Roof type, stairs, steel and dormer design — Existing roof and walls form the starting shell; Garage conversion — Slab, damp, insulation and openings — The shell exists but must become habitable Professional fees, VAT where it applies, kitchens, bathrooms, landscaping and furniture may sit outside a building quote.

Why two storeys cost less per square metre

This surprises people. A two storey extension usually shows a lower rate than a single storey one of the same footprint, because the two most expensive elements of any extension are the ground and the roof. Foundations and a roof serve one floor in a single storey build and two floors in a two storey build, so the fixed cost is spread across double the area.

It does not follow that two storeys are cheaper overall. The total is higher, planning is harder, and the disruption inside the existing house is greater because bedrooms and often a bathroom are affected.

The site factors that move the rate

Before anyone looks at kitchens, the ground and the access set the tone of the price. A tight terraced site in Tunbridge Wells with no side access is a different job from a detached property in rural Kent with a drive a lorry can reverse down.

Access: everything barrowed by hand adds labour to every single delivery · Ground conditions: clay, made ground or a high water table means deeper foundations · Trees: nearby roots frequently force engineered or piled foundations · Drainage: a run diverted or a manhole rebuilt is a day and a permit, not a minute · Levels: a sloping garden means retaining structure before the extension begins · Existing structure: a Victorian solid wall behaves differently from a 1990s cavity wall

What a rate may not include

This is where budgets fail. The build rate is for the building. The following are real, predictable costs that homeowners regularly discover late, and they are worth listing in your own spreadsheet before you commit.

Common items outside an early build rate. Item | What shapes it | When it applies. Architectural drawings — Scope, surveys and design changes — Almost always; Structural engineer — Steel, foundations and design complexity — Whenever structure is altered; Planning application — Application route and supporting reports — Where permitted development does not cover it; Building control — Project scope and inspection route — For regulated building work; Party wall surveyor — Number of owners and whether an award is needed — Shared walls and nearby foundations; Kitchen supply — Cabinetry, worktops, appliances and joinery — Kitchen extensions; Landscaping and making good — Area, access, drainage and finishes — Where the building work disturbs outside areas

How to use the rate sensibly

Use it to sanity check, never to decide. Multiply your floor area by a mid range figure, add twenty per cent for the items above, then add a ten per cent contingency for what the ground reveals. If the result is comfortably within budget, the project is worth designing. If it only works at the bottom of the range with no contingency, redesign before you spend money on drawings.

When quotes arrive, convert each one back into a rate and compare like with like. A cheaper quote with a lower rate is usually buying less: thinner insulation, smaller steels, standard windows, or a scope that quietly excludes making good.

Frequently asked questions

Is a per square metre rate reliable?

As a budgeting tool, yes, within about twenty per cent. As a price, no. It cannot know your ground conditions, access or specification, which are exactly the things that decide the final figure.

Why is the South East more expensive than the national average?

Labour rates, subcontractor availability and site logistics. Published national averages typically sit ten to twenty per cent below realistic South East figures.

Does a bigger extension cost less per square metre?

Usually a little, because setup, scaffolding, welfare and management are spread further. The saving is real but modest, and it disappears if the larger design adds structure or glazing.

Should I hold a contingency?

Yes. Ten per cent is sensible on a new-build extension and fifteen per cent on anything that involves opening up an older property, where what is behind the plaster is unknown until it is not.