Two storey extension cost: the factors behind the price

Why two floors are not double the cost

Foundations, the roof and much of the scaffolding serve both floors. That is why many homeowners compare a two storey extension against building single storey first and adding later. The trade-off is that walls are taller, the structure carries more load, and the extension has to tie into the existing house at two levels.

Foundations and structure

Foundations must carry two storeys of wall, floor and roof. Ground conditions, nearby trees and drains can mean deeper or specially designed foundations. Openings into the existing house on both floors usually need steel beams or lintels designed by a structural engineer.

Rear, side or wraparound position

A two storey rear extension often needs wide openings into the kitchen and bedrooms, while a side extension may involve building close to a boundary, with party wall procedures and access constraints. Each position changes how much of the existing house is disturbed.

Roof design and tie-in

Joining the new roof to the existing one is one of the main design details. Matching the existing pitch, ridge and coverings looks better but can involve more structure than a lower or hipped roof. Roof windows, valleys and lead work add to the cost.

Glazing, layout and fit-out

Large sliding or bifold doors downstairs and generous windows upstairs add cost. What goes inside matters too: a kitchen on the ground floor and a bathroom or en-suite above are the costliest rooms to fit out, while a bedroom and living room are simpler.

• Size and type of glazing
• Kitchen and bathroom specification
• Heating and electrical extensions
• Flooring and decoration on two levels

Internal changes to the existing house

Upstairs, the new rooms often need access from the landing, which can mean moving a bedroom wall or altering the stairs. Downstairs, knocking through and re-planning the kitchen spread the work into the existing home. Budget for making good these areas as well as the new build.

Planning, design and professional fees

Most two storey extensions need planning permission because first-floor additions affect neighbours' light and privacy. Add architectural drawings, structural calculations, building regulations approval and, where relevant, party wall surveyor fees to the budget.

For design and build, see our two storey extension service.

Ways to keep the budget under control

The simplest form is usually the most economical: a rectangular plan, a roof that ties in neatly and window sizes that use standard lintels. Settling the layout, glazing and kitchen and bathroom specification before the quote stage reduces later changes, which are among the most common causes of budgets slipping. Asking for a quote that separates the shell, the fit-out and the work inside the existing house makes it easier to see where money is going and to compare builders like for like. Holding a contingency for anything found below ground or when walls are opened up is sensible on any extension, and particularly on one that ties into the house at two levels. Agreeing how variations will be priced before work starts avoids disputes later.

Common questions