Double Storey Extensions Explained: Cost, Planning and Structure
In short
A double storey extension adds space on two floors at once, typically a larger kitchen or living area downstairs and an extra bedroom or bathroom above. It shares one set of foundations and one roof, which makes it efficient, but in London it usually needs full planning permission.
A double storey extension builds out on the ground and first floor in a single project. Because the foundations, walls and roof serve two floors, the space you gain for each pound spent can be better than two separate projects. It is a popular choice for semi detached and detached houses that need both a family kitchen and another bedroom.
Planning permission
Under permitted development in England, an extension of more than one storey can currently extend no more than 3 metres beyond the original rear wall, must be at least 7 metres from the rear boundary, must have a roof pitch matching the house as far as practical, and must use similar materials. Upper floor side windows must be obscure glazed and fixed below 1.7 metres. Two storey side extensions are not permitted development. In practice, the 7 metre rule and conservation area restrictions mean most double storey extensions in London need full planning permission. Councils look closely at the effect on neighbours’ light and outlook, and at how the extension relates to the original house.
Structure
A double storey extension is a significant structural project:
- Foundations designed for two storeys of load, often deeper near trees or on London clay.
- Steel beams where the existing rear wall is opened at ground and first floor level, carried on padstones or columns.
- Floor and roof structure tied into the existing house.
- Party wall procedures on semi detached and terraced houses.
A structural engineer designs all of this, and building regulations approval covers structure, fire safety, insulation and drainage.
What shapes the cost
We do not publish a price per square metre, because it varies so much with the details. The main factors are:
- Size and the amount of the existing house that is opened up
- Foundation depth and ground conditions
- Steelwork and any structural complexity at the junction with the house
- Glazing, roof lights and doors
- The kitchen, bathroom and finishes specification
- Access, party wall costs and professional fees
An itemised schedule of works after a site visit is the only reliable way to see the real figure.
Double storey extension or loft conversion?
Both add bedrooms, but a double storey extension also enlarges the ground floor and keeps all rooms on familiar levels. A loft conversion keeps the garden intact. Our article on loft conversions versus extensions compares the two in detail.
How we handle double storey extensions on London’s Prestige projects
We manage the full project: architectural and structural coordination, planning, building regulations, party wall, the build and the interior finishes, with one project manager reporting to you throughout. Our Richmond Hill project shows how much a house can gain when new space on several levels is designed together. See our extension service and book a site visit for an honest assessment of what your home and street will allow.
Common questions
Do double storey extensions need planning permission?
In London, usually. Permitted development allows only 3 metres in depth and requires 7 metres to the rear boundary, which many London gardens cannot meet, and side extensions over one storey are excluded.
Is a double storey extension better value than single storey?
The space gained per pound is often better, because foundations and roof serve two floors. The total cost is higher, and planning is harder, so it depends on what space you need.
How long does a double storey extension take?
Once approvals are in place, the build typically runs for several months, depending on size, structure and the level of internal finish.