What this guide covers: double storey side extension cost · UK planning context · what to ask in a quotation
Quick budgeting answer: Extension costs must be compared on a like-for-like scope. A finished room, weather-tight shell and supply-only kit are different products; the model gives broad scenarios, not a contractor quote.
20m² ground footprint × two storeys, illustrative
A 40m² two-storey side example
With a 4m × 5m footprint and two storeys, the model’s UK-average, contemporary-finish, side-extension middle case is £116,000. It assumes normal access and ground conditions, a basic internal finish, 20% construction VAT, professional and approval allowances, and 10% contingency. It does not include an unusually elaborate staircase, fitted bathroom or difficult foundation solution.
Do not equate this example with a contractor’s fixed price. Structural opening sizes, first-floor access, headroom, drainage and roofing differ enormously between properties. For a more general comparison, see two-storey extension costs.
Why the side position matters
Side land can be constrained by a neighbour, shared drive, drainage routes or a narrow gap. On a two-storey scheme, the distance to boundaries and the effect on neighbouring light or outlook can be particularly important in obtaining planning consent. Check the local authority’s requirements early.
Two levels of load and nearby existing footings demand engineered design.
First-floor levels, openings and the position of the staircase need coordination.
A new side roof must tie into the existing house without compromising weatherproofing.
Bathroom and bedroom uses have very different plumbing and finishing costs.
Quote two-storey work as a completed project
Ask builders to price ground and first-floor construction, not just multiply a single-storey quote by two. The footprint may share excavation and roof expenditure, but additional structure, insulation, access equipment and internal alterations still cost money. Mark allowances for steels, heating and room-specific fit-out clearly.
| Ask about | Why it matters |
|---|---|
| Planning constraints | Have roof height, side boundaries and overlooking been assessed? |
| Load path | Are engineer calculations and foundations included? |
| Access to upstairs | What openings, landing changes and fire-safety work are assumed? |
| Roof and finish | Is matching brickwork, roofing, guttering and full decoration included? |
A useful alternative to compare
If the ground floor does not need enlargement, an extension over an existing garage may be worth a feasibility check, but it brings separate structural questions. If the garden layout allows, a rear two-storey addition may simplify some boundary issues. Compare the design outcomes as well as price.
Calculate your own extension budget
Our published model provides broad planning scenarios; it does not replace a measured survey, specialist design or itemised builder quotations.
Calculate my extension price →Common questions
Is a double-storey side extension cheaper per square metre?
It may share some site and roof costs compared with separate single-storey projects, but the final outcome depends on design and condition.
Is planning permission always required?
Not necessarily for every property, but side-extension height and boundary rules are restrictive. Seek a property-specific check from the planning authority.
What does the £116,000 include?
Only the specified 40m² illustrative model scenario with assumed VAT, fees and contingency, not a bespoke room fit-out or structural survey.
Pricing context: Federation of Master Builders; full-project planning scenarios and their assumptions are described in our pricing methodology. For permissions in England, check the Planning Portal and GOV.UK building-control guidance. Rules differ across the UK. All figures and design comments here are general context, not measured quotes or professional advice.