Establish the roof enlargement entitlement
Class B permits certain roof enlargements to eligible houses. It does not provide the equivalent entitlement for flats or maisonettes, and certain houses created through permitted development routes are excluded. Roof enlargement rights are unavailable on designated land, including conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites.
Elsewhere, planning conditions or Article 4 directions may remove the relevant rights. Check the property's planning history with the local planning authority before relying on a standard rear dormer design. Listed status also requires a separate consent assessment where the proposed work affects the building's special interest.
Calculate the cumulative additional volume
The roof space allowance is 40 cubic metres for a terraced house and 50 cubic metres for a semi-detached or detached house. It concerns the additional external roof volume compared with the original roof space, not the floor area or usable headroom inside the proposed room.
Earlier roof enlargements count, including those built by previous owners. A hip to gable alteration and dormer must therefore be considered together, alongside any other relevant additions. Show the calculation geometrically on the drawings. The government's technical guidance explains the cumulative approach and illustrates roof enlargement conditions.
Check the roof slope and highest point
Permitted development does not allow an enlargement beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway. Both parts of that description matter. A slope visible from a road is not automatically the principal slope, but labelling a dormer rear-facing does not settle an unusual corner-plot arrangement.
No part of the enlargement may rise above the highest part of the existing roof. Balconies, verandas and raised platforms are not permitted under Class B. A proposal incorporating an outdoor sitting area therefore needs assessment beyond a basic dormer volume calculation. A Juliet balcony without a projecting platform is different and should be assessed against the applicable conditions.
Dormer loft conversion plans
- Rear flat-roof dormer: the most common type, built across much of the rear slope to give the most internal space
- Pitched or gabled dormer: a smaller dormer with its own little roof, often used where appearance matters more, such as on a front slope
- L-shaped dormer: used on houses with a rear projection, linking a dormer on the main roof to one over the back addition
Draw the eaves junction and materials
The original eaves must normally be maintained or reinstated, and the enlargement set back, so far as practicable, at least 0.2 metres from them. Measure along the roof slope from the outside edge of the eaves. The qualification is not permission to omit the setback merely for convenience. Specific exceptions concern relevant parts of hip to gable work and enlargements joining extension roofs.
The enlargement must not overhang the outside face of an external wall of the original house, except where the relevant enlargement joins the roof of a rear or side extension. Exterior materials must be similar in appearance to the existing house. Show these relationships in sections rather than relying on a general compliance note.
Resolve side windows and planning evidence
A window in a wall or roof slope forming a side elevation must be obscure-glazed. It must also be non-opening unless its opening parts are more than 1.7 metres above the floor of the room containing it. Raising the opening parts does not remove the separate obscure-glazing requirement.
Dormer loft conversion plans should identify the affected slopes, cumulative volume, eaves detail and window restrictions. Lawful development certificate drawings can support formal evidence of compliance. For other conversion types, see loft conversion planning permission. Where a householder application is needed, the authority assesses design and neighbour effects against local policies. Any building control approval for the conversion remains separate from this roof enlargement planning assessment.
This guide is general information only and does not replace advice on your specific property. Rules can change, so always check the current position before starting work.
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Frequently asked questions
Do I need planning permission for a dormer?
Not necessarily. An eligible house may use Class B permitted development if the complete roof enlargement satisfies every limit and condition. Otherwise, a planning application is needed.
Does an earlier hip to gable conversion reduce my dormer allowance?
Yes. Its added roof volume counts towards the same cumulative allowance. Calculate the earlier alteration and proposed dormer together rather than assigning each a separate volume entitlement.
Can a high-opening side window have clear glass?
Not under these conditions. The height exception concerns whether opening parts are allowed. Side-facing windows must still be obscure-glazed even where their opening parts are above 1.7 metres.
Is the eaves setback measured horizontally?
No. The specified measurement runs along the roof slope from the outside edge of the eaves. A horizontal plan dimension alone may not demonstrate the required setback.