Can I build a second storey on my single-storey extension in London?

Why going up is a different project from going out

Adding a floor over an existing extension looks like the simple option: the footprint is already there, the garden stays the size it is, and the kitchen below keeps working while the room above is built. In practice it turns a modest ground-floor addition into a two-storey one, and the rules treat it that way from the start. The planning limits are tighter, the structure has to carry roughly twice the load it was designed for, and the new upper walls sit at bedroom-window height for the neighbours rather than at fence height.

That is why we do not start with a sketch of the new room. We start with the extension you already have: when it was built, how deep it is, what it stands on and how close it sits to each boundary. If you are still choosing between a new two-storey addition and building over the old one, our two-storey rear extension plans page covers the from-scratch version, and our guide to what extension a semi-detached house can take sets the rear, side and loft options side by side.

Does the upper floor need planning permission?

Usually, yes. Permitted development for houses is set out in Class A of the General Permitted Development Order 2015, and paragraph A.1(h) excludes any enlargement of more than one storey that would extend beyond the rear wall of the original dwellinghouse by more than 3 metres, or be within 7 metres of any boundary opposite that rear wall. The government's technical guidance says plainly that the enlarged part could be a new two-storey extension or the addition of a storey onto an existing single storey extension.

The same guidance adds that where a new part is joined to an existing extension, the limits apply to the total enlargement, old and new together. So if your ground-floor extension already runs 4 m, or 6 m under the prior approval route, an upper floor over its full depth cannot be permitted development; only a first floor kept within 3 m of the original back wall could be, and only if the garden is long enough to leave 7 m to the back fence. Many London terraces and semis have gardens too short for that, which is why most of these projects go to the council as a householder application.

Two more conditions apply even when the size works. Any upper-floor window in a side wall must be obscure-glazed and fixed shut unless the opening part is more than 1.7 metres above the floor of the room, and where the new part forms an upper storey on an existing enlargement, its roof pitch must match the original house so far as practicable. A flat-roofed first floor over a flat-roofed kitchen extension rarely meets that second condition on a pitched-roof house.

Conservation areas and flats

On what the order calls article 2(3) land, which takes in conservation areas, the rule is stricter. Class A does not permit any enlargement that would have more than a single storey and extend beyond the rear wall of the original house. A first floor over a rear extension in a conservation area therefore always goes in as a planning application, whatever its depth, and the officer will look at how the addition reads from neighbouring gardens and any public view of the back of the terrace.

Flats and maisonettes have no Class A rights at all, so building up over a ground-floor flat's extension is a planning application from the outset, and many leases also require the freeholder's consent. Houses where an earlier permission removed permitted development rights by condition are in the same position; our check of the council's planning history picks those up.

Can the existing extension carry another floor?

This is the question that decides the cost, and nobody can answer it from the outside. A single-storey extension's foundations were sized for one storey of wall and a light roof. Some builders dug deeper or wider than they needed to, some did not, and the drawings from the original building control file, if they can be found, show what was designed rather than what went in. A structural engineer will usually ask for a trial pit beside the wall to expose the footing, its depth and the soil it sits on; London clay, tree roots and nearby drains all change the answer.

The Building Regulations require a building to carry its loads to the ground safely; and without causing such deflection or deformation as would harm the stability of another building, and an extension counts as building work under regulation 3. If the existing footings are not enough, the usual options are underpinning them, or standing the new floor on its own steel frame with new pad foundations so the old walls carry little extra. The flat roof joists of the old extension are rarely strong enough to become a floor, so expect a new floor structure in most cases. Our structural drawings service covers the calculations, from £745, and our guide to what structural calculations cost explains what the engineer produces.