House Extensions Sidcup
Single storey rear and double storey extensions in Sidcup, planning, structure and build under one contract.
- Rated 4.9 from 31 reviews on MyBuilder
- Single-storey rear extension from £40,000
- Free site survey, itemised quote
GS Renovation builds extensions across Sidcup, where 1930s semis, post-war houses and detached homes sit on plots that suit kitchen, side and rear extensions. We handle foundations, steelwork, roofing and full fit-out, guiding you through permitted development or a planning application to the council.
In Sidcup, a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m² (indicative London ranges, Architecture for London, Resi, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Sidcup: the structural engineer we appoint designs the structure, we build it, and building control signs it off.
Price guide
| Work | Indicative London range |
|---|---|
| Single-storey rear extension | from £40,000 |
| Rear extension, per m² | £2,000 to £2,800 |
| Side return, per m² | £2,400 to £3,200 |
| Wraparound, per m² | £2,400 to £3,200 |
| Double storey, per m² | £1,800 to £2,600 |
| Basement extension | see the basement page |
Indicative London ranges (Architecture for London, Resi, MyBuilder and Checkatrade, 2026), not a quotation. Prices checked September 2026. Full breakdown: Extension Costs & Planning.
Quick estimate for Sidcup
Sidcup range: choose above
Kitchen extensions and side extensions in Sidcup
The wide plots common across Sidcup give real scope for extending, and the most asked-for project is a kitchen extension: a single or double storey rear addition that opens the back of a 1930s semi or post-war house into one large kitchen-diner. Where the house has a side passage or an old garage, a side extension can take a utility room, a downstairs WC or a wider kitchen, and a wraparound joins the side and rear into one L-shaped room. Where the original rear wall comes out, we install a steel beam sized by the structural engineer we appoint to carry the load above.
We start with proper foundations and groundworks suited to the ground conditions, then build up in brick or block to match the interwar character, tie in the new roof, and complete the internal fit-out. Because a semi shares a boundary and party wall with the neighbour, excavating near that line usually brings the Party Wall Act into play. You serve the party wall notice as building owner (or your surveyor does); we give you the drawings and programme it needs and build to any award.
Housing in Sidcup
Take the 15,110 or so homes the valuation list gives Sidcup: 40% flats, 60% houses, 12% from before 1919, 30% interwar and 57% post-war. 27% of homes here are semi-detached; a rear extension on a semi in Sidcup digs beside the attached neighbour, so one party wall notice is usual.
Bexley's new design guide and a Sidcup rear extension
Bexley adopted the Design Guide SPD Part 2, which includes Building Alterations and Extensions guidance, on 29 June 2026, so a Sidcup extension drawn before that date may have been designed to older guidance and is worth checking before it is submitted. For a terraced or semi-detached house the guide says an extension should typically not exceed 3.5m deep. For the attached houses of Sidcup that depth is the starting point for a planning application, and a deeper scheme needs a clear reason on the drawings.
Height is the second test. The guide says a single-storey extension should not exceed 4m at its highest point, with an eaves height of no more than 3m. That caps a lean-to roof against the back wall of a Sidcup semi, and it often points towards a flat roof with a lantern, or a shallow pitch with rooflights. On post-war and 1930s semis a raised garden or a sloping plot eats into those limits fast, so we take levels at the first visit and the drawings start from real heights.
Planning and building control in Bexley
Many single storey rear extensions on Sidcup houses fall under permitted development, meaning no full planning application is required, though this depends on the size, height and whether previous extensions have used up your allowance. A single storey side extension can also be permitted development on a house outside a conservation area, if it is no wider than half the original house and no more than 4 metres high, but a wraparound that joins side and rear usually needs an application. Double storey additions, work to the front, or anything affecting the street scene usually needs planning permission from Bexley Council, and we advise which route applies before any drawings are finalised.
Whichever route the property takes, every extension must satisfy building regulations, and Bexley Council building control inspects the foundations, the steelwork, the insulation and the drainage as the work proceeds. We book those inspections and keep the paperwork moving alongside the build, so you have one team accountable for both from start to finish.
Side return extensions in Sidcup
Side returns belong to Victorian and Edwardian terraces, and only 12% of homes in Sidcup predate 1919, so they are the exception here; most extensions run across the full rear width instead. Our side return extension page covers the build in detail, with build rates of £2,400 to £3,200 per m².
Double storey extensions in Sidcup
Under permitted development a two-storey rear extension can go no more than 3 metres beyond the original rear wall and must stay at least 7 metres from the rear boundary, and in a conservation area it is not permitted development at all. Many gardens are too short for the 7 metre rule, so a double storey extension in Sidcup often needs planning permission from Bexley Council. With 35% of homes here semi-detached or detached, the side of the house is often an option too. The Christ Church Conservation Area appraisal states that dormers or other extensions to the roofscape are likely to adversely affect its established character (council source). Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.
- Flat and pitched roofs with rooflights or lanterns
- Insulation, airtightness and thermal detailing to regulations
- Foundations, groundworks and steel frames
VOA Council Tax stock of properties, March 2025, for Sidcup West, Sidcup East, Lamorbey and Foots Cray Meadows. Counts rounded by the VOA.
Why Sidcup Homeowners Choose GS Renovation
Sidcup homeowners choose GS Renovation because we build extensions that suit Sidcup's 1930s and post-war houses and their generous plots, not off-the-shelf boxes. One team handles the build, from foundations, steel and roofing to the full fit-out, and gives you the drawings for any party wall notice you serve. We also tell you honestly whether your project is permitted development or needs planning permission from Bexley Council. Our founder's 30+ years on the tools stands behind every build.
- Permitted development checked before you pay for a planning application
- Drainage diverted or built over with the right approval in place
- One point of contact from first survey to handover
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
Giorgio is an excellent and intelligent workman, and with very reasonable prices! He installed a kitchen extractor fan in a complicated situation…
George was brilliant. Lovely bloke, very reasonably priced and a perfectionist to boot. He completed our small kitchen renovation with care and great…
FAQ
Frequently Asked Questions about House Extensions Sidcup
What happens to the house extensions price if the scope changes in Sidcup?
The house extensions price in Sidcup moves visibly when the scope changes, because the quotation is itemised within £2,000 to £2,800 per m² for a rear extension (indicative). A change at DA14 is priced against the same lines rather than renegotiated, and any change touching the approvals, such as a Bexley Council building control inspection of the foundations before concrete is poured, is flagged. GS Renovation issues a written variation and waits for approval.
Will my Sidcup rear extension need planning permission or is it permitted development?
Many single storey rear extensions on Sidcup houses are permitted development, so no planning application is needed, but it depends on the depth, the height, how close it sits to boundaries and whether earlier extensions have used your allowance. Double storey or front additions usually need planning permission from Bexley Council. Either way the work must meet building regulations. We assess your property and confirm the correct route before drawings are finalised. A listed building is the exception: work that affects its character, inside or out, including an extension or taking out a wall or chimney breast, needs listed building consent from Bexley Council, and doing it without consent is a criminal offence.
Can I extend a Sidcup flat under permitted development?
No. The permitted development rights for rear extensions apply to houses only, not flats or maisonettes. With flats making up a large share of Sidcup's dwellings, this is a common question. Any extension to a flat needs planning permission from Bexley Council and, under most leases, the freeholder's written consent, as well as building regulations approval.
What happens to the old back wall when my Sidcup house is extended?
It becomes an internal wall with a large opening. That wall carries the first-floor rear wall and joists, so an engineer sizes a steel beam and its bearings, and it is propped while the brickwork comes out. The opening is formed once the new shell is watertight, so the house stays sealed for most of the build.
Where does rainwater from a new Sidcup extension roof go?
To an approved outlet. Downpipes are connected to a soakaway where the ground drains well, or to the existing surface water drain. Part H requires drains laid to proper falls, and building control inspects them before trenches are backfilled. If the extension sits over an existing drain, its route is surveyed first.
Flat roof or pitched roof for a single-storey extension in Sidcup?
Often decided by the windows above. A pitched roof needs height, and it must stay below the first-floor sills, which can make the slope too shallow for tiles. A flat roof avoids that and suits rooflights or a lantern. Either way, the build-up must meet current Part L insulation standards.
Can a Sidcup semi extend deeper than the design guide's 3.5m by another route?
The design guide applies to planning applications. The larger home extension route under permitted development is a separate prior approval process, and Bexley offers a Fast Track fee for a decision within 20 working days of validation. That route has its own depth limits and neighbour consultation, and only applies where permitted development rights survive. It is not available in a conservation area, where side and two storey rear extensions also need planning permission. Confirm the rights first, then compare the two routes.
Will a large Sidcup extension be liable for the Community Infrastructure Levy?
Only above a threshold. Bexley states that development creating a new home or increasing floor space by over 100 square metres is likely to be liable to pay CIL. A single storey rear extension on a Sidcup semi normally sits well under that. A wraparound with a two storey side addition on a detached house can approach it, so add up the floor area on the drawings early, and where CIL applies, claim any exemption before work starts.
Are trees at the back of a Sidcup garden a problem for an extension?
They can be. Bexley states that trees are protected if they are under a Tree Preservation Order or in a conservation area, and removing or damaging a protected tree can bring a fine for each tree affected on conviction. Damage includes cutting roots while digging foundations. Check the tree's status before the design is fixed, and where a protected tree is close, the structural engineer we appoint can design foundations that bridge the roots.
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