Building control

Loft conversions

This section only applies to a loft conversion of no more than two rooms and a floor area of at most 50 square metres in an existing one or two storey single-family dwelling house.

For advice on building types, contact us. 

The information here is for guidance only. Loft conversions often involve complex construction issues; it is best that you consult a qualified architect or surveyor.

Structural stability

A structural engineer should ideally check new structural members or alterations to existing structural members.

The Building Control Surveyor checking the plan or the works on site may require calculations to show that the new structure is adequate or that the work will not adversely affect the stability of the existing building.

Floor joists

Existing ceiling joists are likely to be inadequate as floor joists; therefore new deeper joists will be needed.

Floor joists can be supported on load-bearing walls or floor beams.

Load-bearing walls

Load-bearing walls need to carry the loads directly to an adequate foundation, a beam, or a structural member that will support the load.

Floor beams

Beams to support floor joists can be made of solid timber, steel, or a combination of timber and steel (a flitch beam).

The exact size will depend on the load on each beam and its span.

Roof structure

The existing roof structure may be altered or upgraded as part of the conversion.

Existing rafters, purlins or props may need to be removed to install the new floor or roof windows.

Where purlins or any intermediate props are removed, a new beam may be introduced under the props or purlins where they have been removed, or the size of the floor joists may be increased to carry the load.

In a traditional roof design, the rafters are prevented from spreading by tying the ceiling joists or collars securely to the rafters and using the purlins as beams. If these are cut or removed (e.g. when installing a dormer), then a ridge beam may be required; if this is the case, a structural engineer should be consulted.