Single Storey vs Double Storey Extension: A Direct Comparison

The comparison at a glance

The figures below assume the same ground floor footprint and a comparable specification, using typical South East ranges for 2026.

Single storey versus two storey extension compared. Factor | Single storey | Two storey. Cost per m² — £2,000 – £3,800 — £1,900 – £2,800; Total cost, 25m² footprint — £50,000 – £95,000 — £95,000 – £140,000; Area gained, 25m² footprint — 25m² — 50m²; Build time — 10 – 16 weeks — 16 – 26 weeks; Permitted development potential — Frequently yes — More restricted; Disruption to bedrooms — Minimal — Significant; Party wall likelihood — Moderate — High; Roof works to existing house — Limited — Extensive; Typical value uplift — 5 – 12% — 10 – 20% Ranges are indicative regional figures for construction only and exclude fees, VAT and fit-out.

Why two storeys costs less per square metre

Foundations and roof are the two most expensive components of any extension, and both are largely fixed by the footprint rather than the volume. Build one storey and those costs are spread over twenty-five square metres. Build two and the same costs are spread over fifty.

Scaffolding, site setup, welfare, management and professional fees behave the same way. The result is that the second storey typically adds around seventy to eighty per cent of the first storey's cost while adding one hundred per cent of the floor area.

Where single storey wins

Single storey extensions are easier in almost every non-financial respect. Permitted development rights are more generous, so many rear extensions avoid a planning application altogether. The upstairs of the house is untouched, so you keep your bedrooms and your bathroom throughout. The build is shorter, the scaffolding is lower and the neighbours notice less.

It is also the right answer where the problem is a cramped kitchen and dark living space, which is the most common complaint in British housing. Adding a bedroom you do not need to solve a kitchen you hate is an expensive way round.

Where two storeys wins

If the house is short of bedrooms or bathrooms, a two storey extension is the only version that fixes it. Moving from three bedrooms to four, or adding a family bathroom and an en suite, moves a property into a different search bracket, and that is where the value uplift genuinely comes from.

It is also the better long-term decision if you can only face one building project. Returning in five years to add a first floor onto an existing single storey extension means new foundations checks, a demolished roof, fresh scaffolding and a second round of disruption, and it always costs more than doing it once.

Planning: the practical difference

Single storey rear extensions can often proceed under permitted development, subject to depth limits, height limits, materials and the property not being in a conservation area or previously extended.

Two storey extensions have tighter rules: they generally must not extend more than three metres beyond the original rear wall, must be at least seven metres from the rear boundary, and must not be to the side or front without permission. A great many require a full application, and objections about overlooking and overbearing scale carry real weight against a two storey scheme.

Check whether the property has already used its permitted development allowance · Conservation areas and Article 4 directions remove rights entirely · Two storey side extensions on semis are frequently required to be set back from the front · Overlooking from new first floor windows is the most common ground of objection

Living through it

A single storey rear extension can usually be built with the house occupied, with a temporary kitchen for four to six weeks. It is uncomfortable rather than impossible.

A two storey build takes the roof off part of the house, opens a bedroom to the elements at some point, and typically means a bedroom or bathroom out of use for weeks. Many families move out for part of it, and that cost belongs in the budget from the start.

A simple way to decide

Write down what the house actually lacks, room by room. If every item is on the ground floor, build single storey and spend the saving on glazing, insulation and a kitchen you will enjoy. If two or more items are upstairs, price the two storey version properly before dismissing it, because the second floor is the cheapest space you will ever add to that house.

Frequently asked questions

Is a two storey extension twice the price?

No. It typically costs seventy to eighty per cent more than the single storey version of the same footprint, while adding one hundred per cent more floor area, because the foundations and roof are shared.

Which adds more value?

Two storey extensions generally add more, particularly where they add a bedroom and bathroom. A single storey kitchen-diner adds strong saleability but usually less headline value.

Can I add a second storey later?

Only if the foundations and walls were designed for it, and it is more expensive and more disruptive than building it at the outset. If it is a realistic future plan, tell the engineer now so the foundations are sized for it.

Which is faster to build?

Single storey, typically ten to sixteen weeks against sixteen to twenty-six for two storeys, and with far less impact on the parts of the house you are still living in.