Guide · 14 October 2026

Loft conversions: planning, building regulations and party walls

A loft conversion is one of the most space-efficient projects in a London house. Before any design work, it helps to understand the three sets of rules involved.

1. Planning permission

Many loft conversions on houses fall under permitted development. The Planning Portal sets out the main limits:

• The extra roof space can be up to 40 cubic metres for a terraced house, or 50 cubic metres for other houses (including any earlier roof additions).
• The work can't go higher than the existing roof, and a dormer can't project beyond the roof slope on the front of the house where it faces a road.
• No balconies, verandas or raised platforms.
• Side-facing windows must be obscure-glazed, and if they open, the opening part must be at least 1.7 metres above the floor.

As with extensions, flats, conservation areas and streets with Article 4 directions usually need a full application.

2. Building regulations

Turning a loft into a room always needs building regulations approval, whether or not planning permission is required. Inspectors check structure, fire safety and escape, stairs, insulation and ventilation.

3. Party walls

In a terraced or semi-detached house, cutting into the shared wall for steel beams normally falls under the Party Wall etc. Act 1996. GOV.UK guidance says notice must be served on the neighbour at least two months before work on a party wall starts.

Sources: Planning Portal – Loft conversion; GOV.UK – Building regulations approval; GOV.UK – Party Wall Act guidance.