Nightingale Mansions is a 66 sqm rear extension to a ground floor flat in a period mansion block in SW12, close to Clapham Common. It took 16 weeks to build and cost £85,000 plus VAT. Bought as an investment property, the flat now has an open-plan kitchen, dining and living space with floor-to-ceiling Crittall windows, two architectural roof lights and parquet flooring throughout. Below is the full project: the brief, the design, the floorplan, the cost and timeline, and the finished rooms.
Floorplans, before and after shots and the complete photo set in one document.
Download the Nightingale Mansions case study (PDF)
The project at a glance
| Location | SW12, near Clapham Common |
| Property | Ground floor flat, period mansion block |
| Extension type | Rear extension |
| Size added | 66 sqm |
| Build time | 16 weeks |
| Cost | £85,000 + VAT |
The starting point
Nightingale Mansions is a ground floor flat in a period mansion block in SW12, originally bought as an investment property. Before work started, the rear of the flat was an empty, unfinished room: bare concrete floor, tired French doors, and no real connection to the garden behind. The owners wanted a proper family living space, built to the same standard as a house extension, but working within the constraints of a flat in a shared building.
What we built
The extension adds 66 sqm at the rear, creating one open-plan kitchen, dining and living space that runs the width of the flat. Floor-to-ceiling Crittall windows frame the garden in distinct panelled sections, while two architectural roof lights bring daylight deep into the room. Herringbone parquet ties the whole space together, and two large mirrors on the side wall help the room read wider than it is.
The kitchen sits at one end, with fitted floor-to-ceiling cabinetry and a large island that doubles as a breakfast bar and houses an integrated wine fridge.
The floorplan
The rear extension holds the open-plan kitchen, dining and living area. Off the hallway, the flat keeps two bedrooms (one with an ensuite), a family bathroom and a utility room, so the extension adds living space without disturbing the private rooms at the front.
The ground floor plan is included in the case study PDF (download above).
The design features that make it work
Crittall windows. Floor-to-ceiling Crittall glazing frames distinct sections of the garden rather than one uninterrupted sheet of glass, which suits the proportions of a mansion block flat and gives the room a strong architectural line.
Two roof lights, not one. Splitting the roof glazing into two openings, rather than a single large lantern, spreads daylight more evenly across the room and avoids one glare-heavy hotspot in the middle of the space.
Parquet flooring throughout. Running the same herringbone parquet from the kitchen through to the living area removes any visual break between the three zones, so a 66 sqm extension still reads as one room.
Kitchen island with wine fridge. The island carries the sink, prep space and an integrated wine fridge, and doubles as a breakfast bar, so the kitchen end of the room works as hard as the living end.
Mirrors for scale. Two large mirrors on the side wall bounce light from the Crittall windows and roof lights back across the room, and make the space feel wider than the flat’s actual footprint.
Planning and party structure considerations
This is an important difference from a house extension: flats in England do not have permitted development rights. Any extension to a flat, including a rear extension like this one, needs a full planning application to the local authority, regardless of size. If you’re planning work on a house rather than a flat, our planning checker is a quick way to see where your project might sit.
Because the flat sits within a shared mansion block, the project also needed to consider the Party Wall Act, which covers party structures such as shared floors and walls between flats, not just boundary walls between houses. Read more on our party wall page, and speak to us early if your project involves a flat or converted building.
What it cost and how long it took
The build came in at £85,000 plus VAT over 16 weeks, for 66 sqm of new space finished to a high standard. Costs vary with size, specification and the condition of the existing building, so treat this as one real data point. For more on how rear extension costs break down, see our extension cost guide, or read more on our rear extension page.
From the design team
“This extension we completed is for a ground floor flat within a tall block containing individual flats above. As a result, it is vital to prepare and submit the relevant planning application, even if the proposed design would typically fall within permitted development rights. It is important that a full householder planning application is prepared and submitted by your architect.”
— Steven Davidson, Architect, Build Team
Rear extensions on mansion block flats: common questions
Do flats have permitted development rights?
No. Unlike houses, flats in England do not have permitted development rights, so any extension needs a full planning application, whatever the size.
How much does a rear extension cost in London?
It depends on size and specification. Nightingale Mansions was £85,000 plus VAT for 66 sqm. Our extension cost guide (linked above) breaks the numbers down.
How long does a rear extension take?
This one took 16 weeks on site. Timelines shift with size, ground conditions and specification.
Do I need a party wall agreement for a flat extension?
Almost certainly. In a shared building the Party Wall Act covers party structures such as shared floors and walls, not just boundary walls, so notices are needed before work starts. See our party wall page (linked above).
Thinking about extending a flat?
Build Team is a London design and build specialist, trusted by more than 1,750 London homeowners. We design and build rear extensions like Nightingale Mansions, including the planning route and party structure agreements a flat extension needs, with a clear, fixed price agreed before work begins. Book a free consultation to talk through your home and your budget.
Or call 020 7495 6561 · email hello@buildteam.com
