Foundations to finish, one contractor
Groundworks, structure, roof, services and decoration all come from the same team, so there is no gap between trades for a problem to fall into.
Property Enhancements / House Extensions
House Extensions in London
Rear, side, single and double storey extensions that increase living space and property value, managed from foundations to final decoration.
Get an extension surveyThe side return most London terraces waste
This page is for owners of terraced and semi-detached houses in South and West London who want more ground floor space. K & M Roofing Solutions builds rear extensions, side return infills and double storey extensions, managed from the foundations through to the final coat of paint.
On a Victorian terrace the biggest easy gain is the side return, that narrow strip of yard beside the original rear outrigger which almost every London terrace has and almost nobody uses. Filling it in turns a dark galley kitchen into a full-width open plan room, and because it sits at the rear it usually falls under Permitted Development.
The part that gets underestimated is the roof. Nearly every London extension is finished with a flat roof, often with rooflights set into it, and a flat roof with penetrations through it is exactly where a badly built extension leaks. We are certified flat roofing contractors first and builders second, so on our extensions the roof is the part we are most confident about rather than the part we subcontract.
Benefits
How we build an extension
The roof built by roofers
Your extension roof is built and waterproofed by certified flat roofing contractors rather than subcontracted out, which matters because that roof and its rooflights are where extensions leak.
Planning and party wall handled
Permitted Development checks, planning applications, Building Regulations and the Party Wall Act notices to your neighbours are all dealt with as part of the job.
Built around you living there
Most extensions are built while the family is still in the house, so we seal the works off, keep a working kitchen as long as possible and agree hours before we start.
What we offer
Types of extension we build
Side return infill
The narrow strip beside the rear outrigger filled in to give a full-width kitchen and dining space. The classic London terrace extension, usually Permitted Development, and normally finished with a flat roof and rooflights to keep light in the middle of the plan.
Rear extension
Built out into the garden to create open-plan living or a larger kitchen, without touching the front elevation. Permitted Development allows up to 3 metres on a terrace or semi and 4 metres on a detached house, with greater depths possible through the prior approval process.
Wrap-around extension
Side return and rear extension combined into one L-shaped footprint, which gives the largest single-storey gain available on a terrace and usually needs a planning application rather than Permitted Development.
Double storey extension
Ground floor space plus a bedroom or bathroom above, which works out cheaper per square metre than a single storey because the foundations and the roof are shared across two floors. Almost always needs planning permission.
Extension roofs and rooflights
Warm-deck flat roofs waterproofed with certified liquid systems, GRP or EPDM, with rooflights and lanterns detailed into the covering rather than sealed in afterwards and hoped for.
See flat roof repair and waterproofingTerraces over the new roof
Where the extension roof is meant to be used rather than just looked at, from an upstairs room or from the garden, the waterproofing is specified for foot traffic and the deck is built on top of it as part of the same job.
See roof terraces and deckingStructural openings
Steel beams installed to open up the rear wall between the existing house and the new space, engineer-designed and building control inspected, which is what makes an extension feel like one room rather than two.
How it works
How our extension service works
1
Free survey and feasibility
We measure the site, check the boundary and drainage positions and tell you what size of extension your plot and the Permitted Development rules actually allow. About an hour.
2
Design, drawings and approvals
Architectural drawings and structural calculations, then the planning application if one is needed and the Building Regulations submission. Planning takes about eight weeks where it is required.
3
Party wall notices
Where the work affects a shared wall or foundations near a boundary we serve the Party Wall Act notices on your neighbours, allowing the statutory notice period before work starts.
4
Groundworks and structure
Foundations dug and poured to building control inspection, drainage diverted where needed, then walls up and steels in. Typically four to six weeks and the messiest phase.
5
Roof, glazing and watertight
The flat roof built and waterproofed, rooflights and glazing installed and the structure made watertight. From this point the internal work is unaffected by weather.
6
Fit-out and sign-off
Services, plastering, flooring, kitchen and decoration, then building control final inspection and completion certificate, handed over with the 10-year workmanship guarantee.
Why choose us
Why customers choose K & M Roofing
01
Certified HYDRONYLON applicators, a system most London roofers cannot offer.
02
One team for the whole job, from survey to sign-off.
03
Rated 5.0 by 60 customers on Google.
04
Staged payments against completed phases rather than money up front, so what you have paid always matches what has been built.
05
Transparent pricing with no hidden costs, and a 10-year workmanship guarantee.
Common questions
Frequently asked questions
As an indicative market range for 2026, single storey rear extensions typically run £1,800 to £3,000 per square metre, and that is a market range rather than our quote. A side return infill of around 12 square metres therefore lands somewhere between roughly £22,000 and £36,000 before fit-out, with kitchens, bathrooms and high-specification glazing on top of that. A double storey works out cheaper per square metre because the foundations and roof are shared across two floors. Ground conditions, drainage diversions, the size of the structural opening and the amount of glazing move the figure most. A binding price from K & M Roofing Solutions follows a free survey.
Often not. Permitted Development allows a single storey rear extension of up to 3 metres on a terraced or semi-detached house and 4 metres on a detached house, with greater depths possible through the prior approval process, and side return infills within the height and width limits usually qualify as well. You will need a full application for a wrap-around, for anything two storeys, and for any extension in a conservation area or where an Article 4 direction applies. We confirm which at survey stage, before you spend anything on drawings.
If your extension involves building on or against a shared wall, cutting into it for a steel, or excavating foundations within 3 metres of a neighbour and deeper than theirs, the Party Wall etc. Act 1996 applies and your neighbours must be served formal notice. On terraced and semi-detached London property that is the norm rather than the exception. We serve the notices as part of the job and build the statutory notice period into the programme so it does not delay your start date.
Twelve to twenty weeks on site for a single storey side return or rear extension, and longer for a double storey. Before that, allow two to four weeks for drawings, around eight weeks if a planning application is needed and two months for party wall notices, although those periods overlap. From first survey to finished room is commonly six to nine months, and we set that timeline out honestly at quotation stage rather than after you have committed.
Yes, and most of our customers do. The extension is built outside the existing external wall, so the house stays sealed and usable until the structural opening is formed, which is a short and well-planned phase near the end. The hard part is the kitchen: if the extension is replacing it, expect a temporary kitchen for several weeks. Dust protection, access routes and working hours are agreed before we start.
Almost always a flat roof, because it keeps the extension below the first floor windows and works within Permitted Development height limits. We build warm-deck flat roofs, insulated above the deck so there is no condensation risk in the structure, and waterproof them with a certified liquid system, GRP or EPDM depending on the design. Rooflights and lanterns are detailed into the covering as part of the waterproofing rather than fitted afterwards and sealed around, which is the difference between a roof that stays dry and one that does not.
The extension carries a 10-year workmanship guarantee issued in writing at handover, together with the Building Regulations completion certificate that a future buyer or lender will ask to see. The extension roof is covered by the same guarantee, and where it is finished with HYDRONYLON it also carries the separate 10-year product guarantee issued by HYDRONYLON LTD. Structural calculations are certified independently by the engineer.
Free on-site estimate
Let us talk about your roof
Tell us what is going on and we will arrange a free survey. No pressure, no hard sell, just honest advice and a clear price.
Kamil07593 196505
Mateusz07544 188268