L-shaped dormer drawings in Harrow

A guide from the plan before last

The guide is a supplementary document from an earlier plan period, while the borough's current plan is the Harrow Local Plan 2021-2041, adopted in 2,026. We read the new policies and the old design guide together, since both can be relevant to a roof.

Where the two overlap, we follow the more demanding dimension. That keeps the application consistent with how the council has treated roofs on your road.

Suburban semis with long gardens

Gardens cover 34.7 per cent of the borough's land. Many Harrow homes are semi-detached or terraced houses of the interwar period, and many have a single-storey or two-storey rear addition.

The average home costs 526,493 pounds, and prices moved -2.8 per cent in twelve months, which keeps loft extensions a practical choice.

Designations and decision figures

Harrow has 31 conservation areas and Historic England lists 295 entries. Inside an area the rule is: Class B roof enlargements are not permitted development on article 2(3) land.

The council decided 917 householder applications in the latest year, granted 81.7 per cent and decided 93.7 per cent in time. Lawful development certificates numbered 461 in the latest national return.

Permitted development and the valley rule

Permitted development does not use the council's clearance, but it has its own tests: volume 40 m³ (terrace house) / 50 m³ (any other house) additional roof volume, and this height rule: No part higher than the highest part of the existing roof. At a rear roof join: Eaves-retention and 0.2 m set-back do not apply where the enlargement joins the original roof to the roof of a rear or side extension.

If your L fits the national allowance, a certificate may be enough, though we still often keep the valley clearance as good practice so a later planning application would not clash with it.

Hip-to-gable change and the valley

Many Harrow semis have a hipped roof with a small gabled return at the back, which is where a valley sits. The clearance from that valley is the number that matters, so we locate it by measurement and not from the garden photograph. A hipped semi often needs a hip-to-gable change to give the dormer enough width, and the council reads that change separately from the dormer.

We draw both on one roof plan with the clearance dimensioned, so you can see how much roof is left for the room.

Why we keep the guide beside the drawing

The design guide sets a distance, and distances are easy to check. An officer with a scale rule can verify the clearance in a minute. We dimension it directly on the roof plan and the section, with the guide's own wording quoted in the notes, so the check is quick and the answer is already on the sheet.

Costs and the route after the survey

After the survey we tell you whether the L fits permitted development or needs a planning application, and give a fixed quote for that route. If the clearance leaves too little room for the L you wanted, we offer the nearest shape that works, and say what you give up.

The same sheets then serve building control, with the structure, fire and insulation details added, so one package carries you from consent to construction.

Party walls, stairs and next steps

Neighbours share the roof on many semis, so notice applies: Party structure notice at least 2 months before work. Stairs need: Loft conversions where 2.0 m cannot be achieved: 1.9 m at the centre of the stair, 1.8 m at the side (half width).

A second MCIAT chartered technologist checks each set, and we redraw at no charge until approval. See dormer loft conversion drawings in Harrow, house extension drawings in Harrow, the Harrow architectural drawings hub, or L-shaped dormer drawings in Ealing.