Rear extension drawings in Harrow from £625
Pinner, Stanmore, Kenton and Harrow on the Hill: go deeper than 3 metres and Harrow wants the extra depth set away from your side boundary, two for one.
£625 £475 drawings + £150 survey (£625 total), no VAT to add
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The short answer
Harrow’s Residential Design Guide SPD ties planning permission to permitted development: 3 metres deep on a semi or terrace, 4 on a detached house. Deeper may be allowed in special circumstances, but the extra depth must then be set away from the side boundary by twice the extra depth, the “two for one” rule. Boundary height is capped at 3 metres flat or 3.5 pitched. Our fee is £475 drawings + £150 survey (£625 total), no VAT to add; the council’s fee is separate.
Rear extension drawings
from £625
- Drawings
- £475
- Measured survey
- £150
- Total you pay
- £625
- VAT
- None to add
- A measured survey of the back of the house, the side boundaries and the neighbours’ rear walls and extensions
- A “two for one” layout that shows how far a deeper extension must be set back from each side boundary
- A boundary height check against 3 metres flat and 3.5 metres pitched
- A check of your address against Harrow’s conservation areas and Article 4 directions
- Plans to Harrow’s 1 February 2024 checklist, and the route that fits submitted for you
£475 drawings + £150 survey (£625 total). A starting fee: we confirm one fixed figure after checking your address. The council’s own fee, where there is one, is paid to the council.
Get my fixed feeHow a Harrow rear extension runs
Depth check
We compare your wish list with the 3 and 4 metre baseline and say whether a deeper plan has a special-circumstances case.
Survey
We measure the back, both side boundaries and the neighbours’ rear walls.
Design
You choose depth and layout, with the set-back each needs and the route written down.
Draw and submit
We prepare the pack to Harrow’s checklist and submit it, then answer the case officer.
Harrow’s baseline, and the two-for-one rule
Paragraph 6.59 of the Residential Design Guide, adopted on 15 December 2010, says that where all the other permitted development criteria are met, single-storey rear extensions to a depth of 4 metres on a detached house and 3 metres on a semi-detached or terraced house may not need permission. Where permission is required, the acceptable depth is decided by consistency with permitted development and by site considerations, scale, neighbours’ amenity and the character of the area.
A greater depth may be allowed where the extension is sited away from a side boundary, the rear building line is staggered, the neighbour is sited away from the boundary, or the neighbour already has a deeper extension. Paragraph 6.61 then adds the rule: the additional depth should be set away from the side boundaries by twice the amount of additional depth sought. Chamfering is not normally acceptable.
For example, asking for 5 metres on a semi where 3 is the baseline means 2 metres of extra depth, set 4 metres off the side boundary. That is a real design constraint, and we draw it before you fall in love with a layout.
Height, conservatories and conservation areas
The guide says an extension abutting a neighbour should be at most 3 metres high on the boundary with a flat roof, or 3.5 metres at the mid-point of the pitch for a pitched roof. Conservatories within 3 metres of a boundary would normally need brick flank walls or solid panels to avoid overlooking.
Under Class A permitted development gives 3 or 4 metres, or 6 and 8 through prior approval, but rights are removed in conservation areas and where an Article 4 direction applies. Harrow publishes a residential planning guide for its conservation areas, and we check yours first.
What Harrow asks for, and CIL
Harrow’s checklist of 1 February 2024 gives you 21 days to supply missing information or challenge the request before the application is disposed of. Roof plans are asked for at 1:100 or 1:200 wherever a roof is created or altered, which includes a pitched extension roof.
Harrow CIL is £110 a square metre for residential, on 100 square metres or more of new floor space, so a typical extension is outside it. Mayoral CIL is in band 2.
Rear extensions in Harrow in numbers
657 of 695
Householder decisions by Harrow Council between July 2025 and June 2026 (prior approvals left out) that were made inside 8 weeks.
Source: MHCLG live table PS2, district planning application statistics, July 2025 to June 2026 (GOV.UK)
3 m / 4 m
Depth the SPD gives as the permitted development baseline for a single-storey rear extension: 3 metres semi-detached or terraced, 4 metres detached.
Source: Residential Design Guide SPD, adopted 15 December 2010, paragraphs 6.58 to 6.63 (Harrow Council)
2 for 1
Extra depth beyond the baseline should be set away from side boundaries by twice that extra depth.
Source: Residential Design Guide SPD, adopted 15 December 2010, paragraphs 6.58 to 6.63 (Harrow Council)
3 m / 3.5 m
Maximum height on the boundary next to a residential property: flat roof, and pitched roof at its mid-point.
Source: Residential Design Guide SPD, adopted 15 December 2010, paragraphs 6.58 to 6.63 (Harrow Council)
15 Dec 2010
Date Harrow adopted its Residential Design Guide SPD.
Source: Residential Design Guide SPD, adopted 15 December 2010, paragraphs 6.58 to 6.63 (Harrow Council)
6 m / 8 m
Deepest single-storey extension through prior approval under permitted development, for attached and detached houses.
Source: GPDO 2015, Schedule 2, Part 1, Class A: enlargement of a dwellinghouse (legislation.gov.uk)
1 Feb 2024
Date of Harrow’s current National and Local Validation Checklist.
Source: National and Local Validation Checklist, 1 February 2024 (Harrow Council)
1:100 or 1:200
Scale, with a scale bar, of the roof plan Harrow asks for wherever a roof is created or altered.
Source: National and Local Validation Checklist, 1 February 2024 (Harrow Council)
£110
Harrow CIL per square metre listed for residential development (Use Class C3).
£72.73
Mayoral CIL per square metre for permissions granted in 2026 in Harrow, which is in band 2 (£60 when set in 2019). It applies only where CIL is due at all.
Source: Mayoral CIL: Annual CIL Rate Summary 2026 (London.gov.uk)
Questions about rear extensions in Harrow
What is the two-for-one rule?
Where Harrow accepts more depth than the 3 or 4 metre baseline, the extra depth must sit back from the side boundary by double the extra amount.
Can I get more than 3 metres on a semi?
Possibly, if the extension is away from a boundary, the building line is staggered or the neighbour has a deeper one. Permitted development also allows more through prior approval.
How high can the boundary wall be?
About 3 metres for a flat roof, or 3.5 metres at the mid-point of a pitched roof, where it abuts a neighbour.
Do I pay CIL on a kitchen extension?
Not normally. Harrow’s CIL arises at 100 square metres or an additional dwelling.
See how far your Harrow house can go
Send the address and a photo of the back. We reply with the route and a fixed fee, from £475 drawings + £150 survey (£625 total).