House extension costs compared by extension type
The type of extension changes the cost structure, not just the total. A two storey extension shares foundations and roof across twice the floor area, which is why it often looks better value per square metre than a single storey. A side return adds awkward junctions rather than simple footprint. Understanding those differences early stops you designing something and then discovering it was the expensive route to the same result.
How the types differ
This is about relative behaviour rather than fixed figures, because ground, access and specification differ on every site.
Extension types compared on cost behaviour. Type | Cost per square metre | Main complications. Single storey rear — Higher per square metre — Wide openings, drainage, glazing choices; Two storey rear or side — Lower per square metre — Roof integration, upstairs layout, party wall; Side return — High per square metre — Awkward junctions, narrow access, tricky roof; Wrap around — Mixed — Complex roof and structure, larger steel spans; Over garage — Variable — Existing foundations may need strengthening
The fixed costs nobody escapes
Some costs barely move with size, so they weigh proportionally heavier on small extensions: design work, structural calculations, building control, scaffolding, welfare and site setup, and the connection points into the existing house.
This is why a very small extension can feel poor value per square metre, and why stretching a design slightly sometimes costs less per metre than people expect.
Where the existing house bites
Extensions interact with what is already there, and older properties bring their own conditions.
Shallow or unknown existing foundations that need matching or underpinning · Drains running through the proposed footprint · Solid walls with no cavity, changing how the junction and insulation are detailed · Roof lines that do not lend themselves to a clean tie in · Existing heating capacity that cannot serve the extra rooms
Comparing quotes properly
Three quotes are only useful if they price the same job. Give every builder the same drawings, the same specification and the same list of what you expect included, then compare line by line.
A materially cheaper quote usually means something has been excluded or assumed. Find out which before deciding, because those items reappear later as extras.
Budgeting the whole project
Build cost is one part of it. Add professional fees, approvals, kitchens or bathrooms, flooring, decoration, and the cost of living around the works, whether that is takeaways for six weeks or renting somewhere.
A sensible contingency is not pessimism. On extensions, the unknowns are underground and inside walls, and they are only revealed once work starts.
Frequently asked questions
Is a two storey extension cheaper per square metre than single storey?
Usually yes, because the foundations and roof serve twice the floor area. The total cost is higher, but the cost per square metre of space gained is often lower.
Why are side return extensions expensive for their size?
They involve awkward junctions with the existing house, complicated roof detailing and often restricted access for materials, all of which add labour without adding much floor area.
How much contingency should I allow for an extension?
Enough to absorb ground and drainage surprises, since those are the items nobody can price accurately before excavation. Discuss a realistic figure with your builder for your property.
Do extension quotes usually include the kitchen?
Often not. Kitchen units, appliances, flooring and decoration are commonly separate, so confirm what is in and out before comparing prices.
Will I need a structural engineer?
Almost always where walls are removed or wide openings formed, since calculations are needed for the steelwork and for building control approval.
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