Terraced houses
An eligible terrace has a Class B roof-enlargement allowance of 40 cubic metres above the original roof space, including previous enlargements. An end-of-terrace position does not automatically make the house a semi-detached property for this rule: the statutory definition of a terrace concerns a house in a row of three or more houses. [1]
The design should show how new roof work meets the neighbouring buildings and how beams will be supported. Cutting into or raising a party wall can engage the Party Wall etc. Act 1996. Being attached is a reason to investigate the shared construction, not a rule that every loft conversion needs identical notices. [4]
Where there is a rear roof or outrigger, an L-shaped dormer may be a design option. Its two parts must be considered together for the roof-enlargement assessment; the rear section is not a separate allowance. [1]
Semi-detached houses
For eligible houses other than terraces, Class B permits a cumulative roof-volume increase of up to 50 cubic metres, subject to all its other limitations and conditions. Where a semi has a hipped end, a hip-to-gable alteration may create additional space; whether it works structurally and visually still needs assessment. [1] [5]
Do not copy the other half of a pair without checking its circumstances. Its work may have used planning permission, different original construction or an earlier version of the rules. The relevant assessment is the proposed work to your house against its own original roof and current constraints. [1]
Bungalows
A bungalow conversion needs a new staircase where none previously served an upper storey. Test the impact on the ground-floor layout as well as the room gained above it. Both the new opening and the stair's finished headroom need to be designed with the floor and roof structure. [6] [7]
The fire approach can differ from a conversion above a two-storey house. Approved Document B distinguishes upper floors no more than 4.5 metres above ground from accommodation above that height. A qualifying lower upper floor may use an escape-window arrangement, whereas a higher floor generally needs a protected stair or another compliant solution. Measure the floor height; do not assume every bungalow conversion falls below the threshold. [3]
A common feasibility checklist
For any of these house types, ask the design team to establish:
- The existing roof form and supporting construction, including the effect of new loads below the loft.
- A stair and landing layout with finished headroom.
- The escape route through the completed house.
- The correct planning route and any previous roof enlargements.
- Whether work to shared structures requires party wall procedures. [2] [6] [3] [1] [4]
A flat within a converted terrace is still a flat, not a house with householder roof-development rights. Treat its permissions and ownership as a separate assessment. [8]