Extension and loft conversion drawings for London homes

Which extension shapes do London houses use?

Four shapes cover most London houses, and each is listed below with the fee at which its drawings start. Each is drawn against the survey with the neighbours’ windows in view, because how far it reaches and how high it rises are among the things a council weighs. Permitted development covers some of them, within size limits and subject to your address; the rest need householder planning permission.

• Rear extension: a room added across the back of the house. Drawings start at £475 drawings + £150 survey (£625 total).
• Side return extension: fills the narrow passage beside the back of a terrace, often along the boundary. Drawings start at £515 drawings + £150 survey (£665 total).
• Wraparound extension: the rear and the side return built as one. Drawings start at £855 drawings + £150 survey (£1,005 total).
• Double-storey extension: adds a room above as well as below. Drawings start at £755 drawings + £150 survey (£905 total).

• Party wall notices for work on the boundary

How do loft conversion drawings differ?

A loft is drawn from the inside as much as the outside: the new stair, the floor structure and the shape of the roof matter as much as the view from the garden. Five loft types are listed below, with the fee at which drawings start where one is published.

• Velux loft conversion: rooflights set into the existing roof slope, leaving the roofline as it is. Drawings start at £475 drawings + £150 survey (£625 total).
• Dormer loft conversion: a projecting structure on the roof slope that adds headroom and floor space. Drawings start at £475 drawings + £150 survey (£625 total).
• Mansard: replaces the roof slope with a steeper lower pitch and a flatter top. Drawings start at £615 drawings + £150 survey (£765 total).
• L-dormer: a dormer that wraps over the main roof and a rear outrigger. Quoted after the address check.
• Hip-to-gable: builds the sloping side of the roof up into a vertical gable end. Quoted after the address check.

• Planning permission drawings
• Building regulations drawings and structural calculations

In the model: The glazed roof. A glazed roof then closes the top of the side return, leaving the new room open to the sky.

What about basements and garden rooms?

A basement is the largest job here, starting at £1,705 drawings + £150 survey (£1,855 total) for a cellar conversion. Digging down changes the foundations, so the sheets typically deal with underpinning, waterproofing and any lightwell, and the structural calculations carry more weight. Some London councils also have specific basement policies, which we read before drawing.

A garden room is the smallest job, from £475. Whether it needs an application depends on its size, its position and your address, and flats and conservation areas change the answer, so the address check comes first.

• Every extension, loft and basement fee

What is the 45-degree rule for extensions?

The 45-degree rule is a daylight check that councils often use to judge whether an extension would crowd a neighbour’s window. On the plan, a line leaves the middle of the neighbour’s rear window at 45°, and the test looks at how the new building sits against that line. The 25° test is a related check.

Your council’s own guidance decides which tests it applies and how it reads them. We record the neighbours’ windows at survey stage, so the drawings show early, while the design can still change, if a proposal crosses a line.

• Measured building surveys

In the model: The rear dormer. A rear dormer rises from the roof above, adding a loft room, so both projects can be read as one stacked change to the house.