Single storey or two storey extension: what changes the answer

Do you need more bedrooms as well as living space

If the household only needs a bigger kitchen or a garden room, a single storey extension delivers that without the added structural and planning complexity of a second floor. If you also need another bedroom or bathroom, building the second storey now, while the foundations and scaffolding are already committed, is usually more efficient than adding it later as a separate project.

Foundations and structure have to support the load

A two storey extension carries more weight and needs a wall structure capable of supporting the upper floor, which is why foundation design has to account for the full height from the outset, not just the ground floor. Retrofitting a second storey onto foundations only ever designed for one storey is rarely straightforward and often means substantial extra work.

If there is any chance you will want to add upstairs later, say so at design stage so foundations can be sized to allow for it, even if you build the upper floor in a later phase.

Planning implications of extra height

A two storey extension is more likely to affect a neighbour's light or overlook their garden, which raises the chance of needing full planning permission rather than falling under permitted development, and increases the likelihood of a neighbour objection. Boundary distances and window positions on the upper floor need particular care.

Single storey extensions have generous permitted development allowances in most cases, though depth, height and proximity to boundaries still apply.

Garden and light impact

A taller structure closer to the boundary can reduce light reaching neighbouring gardens and windows, which is one of the main things a planning officer or objecting neighbour will raise. A single storey extension with a modest roof height has a much smaller impact on light and outlook for anyone next door.

Cost per square metre of usable space

Because the foundations, scaffolding and roof structure are largely shared costs regardless of how many storeys sit on top, a two storey extension typically delivers more usable floor area for each pound spent on those shared elements than a single storey extension of the same footprint. That does not mean it is cheaper overall, only that the second storey is often better value than building it as a standalone project later.

Common questions