Double-storey extension drawings in Hillingdon from £905
Uxbridge, Ruislip, Hayes and Northwood: the council wants the upper floor within a 45 degree line of sight and set well down from your main ridge.
£905 £755 drawings + £150 survey (£905 total), no VAT to add
Prefer WhatsApp? Send us a message · +44 7592 201353
The short answer
Hillingdon’s HDAS Residential Extensions SPD, as the council quotes it in committee reports, says two-storey extensions must meet the 45 degree line of sight, must not exceed the height of the existing building and should have roof lines parallel to it. A 2018 report approved a first-floor extension 3.6 metres deep under a hipped roof set 1.8 metres below the main ridge. Our fee is £755 drawings + £150 survey (£905 total), no VAT to add; the council’s fee is separate.
Double-storey extension drawings
from £905
- Drawings
- £755
- Measured survey
- £150
- Total you pay
- £905
- VAT
- None to add
- A measured survey of both floors at the back and the neighbours’ rear windows
- The 45 degree line of sight drawn from the nearest neighbouring windows
- A roof that stays parallel to your main roof and well below the ridge
- A check of your address against Hillingdon’s 31 conservation areas
- Plans to the February 2024 validation checklist, and the route that fits submitted for you
£755 drawings + £150 survey (£905 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 Hillingdon double-storey extension runs
Line of sight
We draw the 45 degree line from the neighbours’ windows before you fix a depth.
Survey
We measure both floors, the ridge and the neighbours’ rear windows.
Design
You choose layout and roof, with the ridge drop and the route written down.
Draw and submit
We prepare the pack to the checklist and submit it.
What the council says it applies
In its March 2018 committee report on a part two-storey extension at 74 Long Lane, the council summarised its HDAS guidance: extensions up to 3.6 metres deep are acceptable on semi-detached houses if they appear subordinate and cause no significant overshadowing, loss of outlook or daylight. Two-storey extensions are subject to the 45 degree line of sight and must not exceed the height of the existing building, with roof lines parallel to it.
The case shows what that looks like in practice. The upper floor projected 3.6 metres under a hipped roof parallel to the main roof, set about 1.8 metres below the ridge, which officers called a sympathetic and subservient addition. For example, an Eastcote semi with a similar hipped drop has a case; a gabled block at ridge height does not.
Permitted development and conservation areas
Hillingdon’s Article 4 direction, due to take effect on 22 September 2014, removed permitted development for single-storey rear extensions over 4 metres deep. Permitted development allows more than one storey only up to 3 metres and not within 7 metres of the rear boundary, so a two-storey scheme is nearly always a householder application.
Hillingdon has 31 conservation areas, and a heritage statement is needed on a householder application in one. We check your address first.
Validation and CIL
For validation, Hillingdon’s February 2024 checklist asks for an existing and proposed block plan at 1:500 and floor plans and elevations at 1:50 or 1:100.
Hillingdon CIL for permissions granted in 2026 is £160.43 a square metre, on 100 square metres or more of new floor space, so most double-storey extensions on a house are outside it. Mayoral CIL is in band 2.
Double-storey extensions in Hillingdon in numbers
637 of 678
Householder decisions by Hillingdon 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)
45°
Line of sight a two-storey extension must satisfy under the HDAS guidance quoted in the council’s report.
Parallel roof
A two-storey extension must not exceed the height of the existing building, with roof lines parallel to the existing roof.
1.8 m
How far below the main ridge the council noted a 2018 first-floor extension’s hipped roof was set.
3.6 m
Depth of the first-floor part of the 2018 extension, and the single-storey depth the guidance accepts.
4 m
Depth beyond which Hillingdon’s Article 4 direction removes permitted development for a single storey rear extension.
31
Designated conservation areas in Hillingdon, from village centres to planned residential estates.
1:500
Scale of the existing and proposed block plan Hillingdon asks for with a householder application.
Source: Local Planning Validation Checklist, February 2024 (Hillingdon Council)
£160.43
Hillingdon CIL per square metre for residential permissions granted in 2026, from a £95 rate indexed by 400/237.
£72.73
Mayoral CIL per square metre for permissions granted in 2026 in Hillingdon, 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 double-storey extensions in Hillingdon
What is the 45 degree line of sight?
A line from the neighbour’s nearest window, at 45 degrees on the plan, that the upper floor must not cross to protect their daylight and outlook.
How high can the roof be?
No higher than the existing building, with a roof parallel to it. The council noted one hipped roof set 1.8 metres below the ridge.
Can I have a flat roof upstairs?
The guidance asks for roof lines parallel to the existing roof, so a pitched or hipped roof is the safer design.
Do I pay CIL on a double-storey extension?
Not normally. It applies from 100 square metres of new floor space or a new dwelling.
See what your Hillingdon house can take upstairs
Send the address and a photo of the back. We reply with the route and a fixed fee, from £755 drawings + £150 survey (£905 total).