Constructum Code builds loft conversions across London as main contractor, for private clients and on architect-led projects. We work to your architect’s and engineer’s drawings and deliver the build through to final handover, with one team accountable for the structure, the trades, the programme and the approvals. Where a client has not yet appointed a practice, we can make introductions.
A loft is structural work before it is a room. The new floor has to carry its loads down through walls that were not built to take them, the staircase has to fit a house that was never planned for one, and a third storey changes the fire strategy throughout. Getting that right is what decides whether the finished space reads as part of the house or as a room in the roof.
Interested in Loft Conversions in London?
The five kinds of loft conversion
Which one your roof can take is decided by its ridge height, its structural form, and how much permitted development volume the property has left.
1. Rear dormer conversions
A rear dormer projects vertically out of the back slope of the roof, replacing pitched ceiling with flat, and turning a triangular void into a room with square walls and standard head height. It is the most efficient way to get a double bedroom and an ensuite out of a loft, and it is what most terraced and semi-detached houses in London end up with.
Because the dormer sits at the rear, it usually falls within permitted development, subject to the volume allowance and matching materials.
2. L-shaped dormer conversions
On a Victorian or Edwardian terrace with a rear outrigger, the dormer can be built across the main roof and along the outrigger in one L-shaped form. That produces two rooms on the top floor rather than one — typically a bedroom with an ensuite, plus a study or second bathroom over the outrigger.
It is the largest conversion available to most terraces, and the junction where the two sections of dormer meet needs proper detailing; it is the point where these conversions most often develop problems later.
3. Hip-to-gable conversions
Semi-detached and end-of-terrace houses, particularly interwar ones, often have a hipped roof that slopes in on the side as well as the front and rear. Hip-to-gable rebuilds that sloping side as a vertical gable wall, which reclaims the full width of the roof.
It is usually combined with a rear dormer, because the two together give enough floor area and head height for a staircase that lands sensibly and a room with usable eaves storage either side.
4. Mansard conversions
A mansard rebuilds the roof into a near-vertical rear wall, set at around 72 degrees, with a flat top. It delivers more internal volume than any other option, and it is often the conversion that conservation officers will accept where a dormer would be refused, because the form is part of the existing London streetscape.
It requires full planning permission and it is the most substantial structural work of the five, since most of the existing roof is removed. Finishes are usually slate, brick slips or zinc to match the terrace.
5. Rooflight conversions
Where the ridge height is already generous, the loft can be converted without altering the roof profile at all: the floor is reinforced, insulation and fire protection are brought up to standard, and rooflights are fitted flush into the existing slope.
It is the quickest route to a usable room and the least disruptive, though the sloping ceilings mean the usable floor area is smaller than a dormer would produce. As a certified VELUX installation partner, we install their roof window systems to the manufacturer’s specification.
Where a house extension is also under consideration, it is generally worth delivering both within one programme.
How we work
Survey first. Ridge height measured from the ceiling joists to the underside of the ridge, roof structure and pitch, chimney stacks, water tanks, the load-bearing walls beneath, and access for scaffolding. This is what determines which of the five conversions your roof can take, and whether the staircase can land where you want it.
Approvals. Planning, or a lawful development certificate confirming the conversion falls under permitted development, is secured by your architect or planning consultant before the build begins. From there we coordinate the building control route, stage inspections and the notifications the works require, and work with the structural engineer and the party wall surveyor where one is appointed, in line with current Building Regulations.
Costs agreed upfront. You receive a detailed, itemised estimate before work begins. Where something changes on site — which on period roofs it occasionally does — it is priced, explained and agreed with you before the work proceeds.
One team, one programme. Scaffolding, structural steel into the party walls, new floor joists, dormer or mansard framing, weatherproofing and glazing, then the staircase, first fix, fire protection, plastering and second fix. On most residential projects this is delivered by the same site team and project manager from start to finish. 95% of our projects are completed on or before the agreed schedule.
Regular communication. Progress updates throughout the build, and direct contact with the project manager running your site.
Minimal disruption. Most of the structural work is reached from the scaffolding rather than through the house, so the loft is framed and weathertight before the ceiling below is opened. Noise, dust, waste and deliveries are managed around the fact that the house remains occupied and the neighbours remain next door.
Guaranteed on completion. A 10-year warranty covering the structural work, and a 12-month guarantee on first fix — the pipework, cabling and carpentry installed before plastering.
Fully insured and accredited. We carry £10m employers’ liability, £5m public liability and £1m professional indemnity cover. Constructum Code Ltd is a TrustMark registered firm, a certified VELUX installation partner for roof window systems, and registered with Certass and FairTrades.
See the loft conversions we’ve completed
What clients say
I’m very happy with the loft conversion and kitchen extension Constructum Code completed at my house in Bromley.
— James Anderson, Bromley
What a loft conversion costs
Loft conversions in London with Constructum Code start from £55,000 plus VAT.
That figure reflects a standard rear dormer conversion on a terraced property, with adequate existing ridge height and workable scaffold access.
The main variables are:
- Conversion type. A rooflight conversion and a mansard sit at opposite ends of the range, because one leaves the roof alone and the other replaces it.
- Existing ridge height. Where head height falls short, either the first-floor ceilings come down or the ridge goes up, and both add cost before the conversion itself begins.
- Structural requirements. Span, steel sizing and how the loads transfer into the party walls. Older roofs with cut timbers rather than trusses can behave differently once opened up.
- Where it lands, whether it takes space from an existing bedroom, and whether it is standard or purpose-made joinery.
- Bathroom and services. Running soil, water and electrics up an additional storey, and whether a pump is needed for drainage.
- Finish level. Joinery, sanitaryware, glazing specification and built-in storage in the eaves.
We price after the survey, so the estimate reflects the actual conditions in your roof.
Common questions
How much head height do I need?
As a working rule, around 2.2 metres measured from the top of the ceiling joists to the underside of the ridge. That allows for the new floor build-up and still leaves the finished headroom Building Regulations require over the staircase — 1.9 metres at the centre of the stair width, reducing to 1.8 metres at the outer edge under the slope. Below 2.2 metres a conversion is still possible, either by lowering the ceilings beneath or raising the ridge, but both change the scope and the cost. We measure this at the survey before anything else is discussed.
Do I need planning permission?
Often not. Rear dormer and hip-to-gable conversions usually fall under permitted development, within a volume allowance of 40 cubic metres for terraced houses and 50 cubic metres for semi-detached and detached, provided materials match and the work stays off the front roof slope. Mansards need full planning permission. Conservation areas, Article 4 directions and flats change the position entirely. Your architect or planning consultant will confirm where your property stands before the design is developed.
What happens to fire safety once the house is three storeys?
Adding a storey brings the house under a stricter part of Approved Document B. That means a protected escape route from the loft down to the front door, fire-resisting doors to the rooms off it, a fire-resisting floor between storeys, and mains-powered interlinked smoke alarms throughout. This is not an optional upgrade and it affects the layout, which is why it is designed in from the start rather than resolved at the end.
Will I need a party wall agreement?
If steel bears into a shared party wall, or a parapet is raised on the boundary, then yes — and on a terrace that covers almost every conversion. Notice is served on the adjoining owners two months before work starts. If they consent in writing, the process ends there. If they dissent or do not respond, surveyors are appointed and issue a binding award. A dissent does not prevent the project going ahead, but it adds time, which is why notices are best served early.
Can we stay in the house during the build?
In most cases, yes, and more comfortably than during an extension. The steel, the framing and the weatherproofing are all reached from the scaffolding, so the loft is closed in before anything opens into the house. The disruptive stage comes later, when the ceiling is opened for the staircase and the services are connected. That period is identified in the programme so you can plan around it.
How long does it take?
A rear dormer or rooflight conversion is typically six to ten weeks on site. Hip-to-gable, L-shaped dormer and mansard projects run longer, with mansards the longest of the five. Before site start, allow a further six to eight weeks for structural calculations, the lawful development certificate or planning application, and party wall notices. A programme is issued alongside the estimate.
Do you work with our architect?
Yes. We build to drawings produced by your architect and structural engineer, and work alongside them through to completion, coordinating the trades and the approvals process around their design. Where a client has not yet appointed a practice, we can introduce ones we have worked with.
Tell us the address and what you are looking to achieve, and we will confirm what your roof can take, what route it takes through planning, and what it is likely to cost.
