Two storey extension plans for houses across London

Can you build a two storey extension without planning permission?

Sometimes, and the conditions are narrow. A two storey rear extension can be permitted development on a house, but only if every one of the following holds — and in London the seven-metre rule is the one that usually fails.

• Maximum 3 metres deep beyond the original rear wall. There is no larger prior approval route for two storeys; the 6 and 8 metre allowances apply to single-storey extensions only.
• At least 7 metres between the new rear wall and the rear boundary of the property. On a typical inner-London terrace with a 12 metre garden, a 3 metre extension leaves 9 metres and passes; on a 9 metre garden it fails outright.
• Roof pitch matching the existing house as far as practicable, and eaves height matching the existing eaves.
• No extension beyond a side elevation fronting a highway, and no more than half the width of the original house on any side extension element.
• Materials of similar appearance to the existing house, and no verandas, balconies or raised platforms.
• Side-facing upper windows obscure-glazed and non-opening below 1.7 metres above the floor.

None of it applies to flats or maisonettes, which have no permitted development rights, to listed buildings, or where an Article 4 direction has withdrawn the right — which across Camden, Islington, Haringey, Hackney and Waltham Forest covers a great many streets.

How will a two storey extension affect your neighbours?

Two storeys is where residential amenity stops being a formality. At ground floor a rear extension mostly affects the neighbour’s garden outlook; at first floor it affects the light reaching their first-floor windows and it creates new windows overlooking their garden. Boroughs test both.

The 45-degree rule is applied in plan and section from the midpoint of the nearest habitable-room window on the adjoining property. At first floor the section test is the harder one, and it is why a two storey extension is often approved at 2.4 metres deep when 3 metres is refused. Some boroughs apply a stricter 25-degree test in section from the centre of the ground-floor window.

Overlooking is the second test and is usually resolved on the drawing. Side-facing first-floor windows to habitable rooms should be obscure-glazed and restricted to top-opening above 1.7 metres. Rear windows are generally acceptable where the separation to the facing properties is conventional for the street, and problematic where the extension pushes into a shorter garden than its neighbours.

On a terraced or semi-detached house, a two storey extension will almost always bear on the party wall, which brings the Party Wall etc. Act 1996 into play alongside planning. Notice for party structure work is one month; for excavation within 3 metres of a neighbouring structure below its foundation depth it is two months. Both run independently of the eight-week planning clock and should start early.

What does a two storey extension demand structurally?

More than a single storey, and in ways that change the budget rather than the drawings. Every element is carrying two floors and a roof rather than one floor and a roof.

• Foundations — sized for the increased load, and on London’s shrinkable clay taken to 2.5 metres or deeper where mature trees are within influencing distance. Underpinning the existing foundation at the junction is common.
• Steel frame at ground floor — opening up the existing rear wall across two storeys usually needs a goalpost frame rather than a simple lintel, with padstones and sometimes new pier foundations.
• First-floor structure — joists spanning onto the party wall need the party wall surveyor’s award to permit the bearing, and the engineer’s design to suit it.
• Fire strategy — a two storey extension frequently creates an inner room at first floor, or encloses an existing escape window. Where the house becomes three storeys in effect, the protected stair requirement under Part B applies to the whole house, not just the new part.
• Thermal continuity — the junction between the new and existing envelope at two levels is where cold bridging and condensation risk concentrate, and Part L compliance depends on detailing it rather than assuming it.

We coordinate the structural engineer’s design with the drawings so the beam sizes on the plan are the sizes in the calculations, and produce the fire strategy as part of the building regulations set rather than leaving it to a site inspector.

What do two storey extension plans cost?

For a two storey extension the Complete package at £1,750 is the sensible choice, because the project will need the building regulations set before construction without exception, and producing both stages from one survey avoids paying twice for the same existing-condition information.

Alongside it, budget the £258 householder application fee or £129 certificate, building control charges of roughly £800 to £1,400 for a two storey scheme, a structural engineer at £600 to £1,200 given the frame and foundation design, and a party wall surveyor at £700 to £1,500 per award where the adjoining owner dissents.

Build costs at 2026 London rates run roughly £2,400 to £3,200 per square metre for a two storey rear extension — slightly higher per metre than a single storey because of the frame and foundations, but delivering twice the floor area for one set of both. A 3 metre deep extension across a 5 metre wide terrace yields around 30 square metres over two floors, which is commonly a bedroom and bathroom above a widened kitchen.

Two storey extension questions