2 Storey SIPs Extension

How the two storeys go together

Ground-floor wall panels stand on the sole plate exactly as they would for a single storey. A floor cassette then sits on top of them: engineered timber joists, usually I-joists or open metal-web joists, with a structural deck. The upper wall panels fix down through that deck, and roof panels close the box.

The engineer cares about three things. First, vertical load: the floor and upper walls add weight to the ground-floor panels, which is rarely a problem, and to the posts beside any wide opening, which often is. A four-metre set of sliding doors under a bedroom wall normally needs a steel or glulam beam and a proper post at each end. Second, racking, the tendency of a tall box to lean in wind. Panels resist it well, but long runs of glass do not, so some solid wall has to stay. Third, the junction with the house.

Where new meets old

Timber panels shrink a little as they dry and settle under load. Masonry does not. Over two storeys the difference can be several millimetres, so the extension is tied to the house with fixings that allow vertical movement, and the roof flashing and any render joint are detailed to slide.

Upstairs, a doorway is cut through the existing wall and needs a lintel. Think about where it lands. Reaching the new room through an existing bedroom makes that bedroom a corridor, and it can also complicate the fire escape route.

Planning: the limits are tighter upstairs

In England, a two-storey rear extension can be built under permitted development only if it meets every one of these conditions in the government's technical guidance:

• it projects no more than 3 m beyond the rear wall of the original house;
• 7 m or more is left between it and the rear boundary;
• its eaves are no higher than 3 m if any part is within 2 m of a boundary;
• the roof pitch matches the house as far as practicable;
• upper side windows are obscure-glazed and fixed shut below 1.7 m;
• the outside materials look similar to the house;
• the house is not in a conservation area, National Park or similar designated land.

Two of those knock out most SIPs schemes. On a semi or a terrace the extension is nearly always within 2 m of a side boundary, and a 3 m eaves limit means no second storey. And a timber-clad box does not look similar to a brick house. In practice, plan on a householder application: the fee is £548 in England and a decision usually takes eight weeks. There is no prior-approval route for larger two-storey extensions, and side extensions with two storeys always need permission.

Scotland sets its own permitted development limits and Wales has its own version too, so read the national guidance for your address.

Fire, sound and escape

Approved Document B asks for 30 minutes of fire resistance from the new floor and the walls that hold it up. SIPs reach that with plasterboard linings fixed to the panel maker's tested detail, which is one reason the lining specification is not yours to improvise.

An upstairs bedroom needs an escape window: a clear opening of at least 0.33 m², no dimension under 450 mm, with the bottom of the opening no more than 1,100 mm above the floor. Any wall within one metre of the boundary must resist fire from both sides and is limited in how much glass it can have.

The floor between a new bedroom and the room below should also hold back sound. A timber cassette does that with mineral wool between the joists and a heavy enough ceiling board.

On site

Expect scaffolding on three sides and a crane or telehandler for the floor cassette and roof. A trained crew can have both storeys standing and wrapped inside two weeks. The slower work follows: roofing, cladding, glazing, the stair or doorway through, and two floors of first and second fix.

Foundations carry more than they would for one storey but still far less than masonry would impose. The foundations page explains why depth is set by the ground. If you share a wall or build close to a neighbour, the Party Wall Act notice periods need to start early.