Key Takeaway

We run fully managed loft conversions across South East London and Greater London, covering everything from structural calculations and council approvals through to steel beam installation, staircase design, and the final interior finish. Because the design and the build sit with one team, there’s no gap where a technical drawing doesn’t quite match what’s actually on site. We work to UK Building Regulations for fire safety and thermal efficiency, keep pricing visible at every stage, and back every finished conversion with a 10-year structural warranty.

Key Facts

According to the UK Planning Portal (2026), loft conversions under Permitted Development allow up to 40 cubic metres of additional roof volume for terraced properties and 50 cubic metres for detached and semi-detached houses without a full planning application. For homeowners in South East London, this means most rear dormer and hip-to-gable conversions can proceed quickly under a Lawful Development Certificate.
Under Approved Document K of the Building Regulations, converted lofts are allowed a reduced staircase headroom of 1.9 metres at the centre of the stair width, dropping to 1.8 metres at the outer edge beneath the sloping ceiling. For period properties with an existing ridge height of 2.2 metres or more, this usually means a compliant staircase can be built without lowering any ceilings below.
Based on our own project tracking across completed residential conversions in Greater London, we hold a 95% on-time completion record, largely because we fabricate bespoke structural steels (RSJs) and custom staircases before scaffolding ever goes up on site.
Every loft conversion we complete comes with a 10-year structural warranty covering the structural steel beams, subfloor joist networks, dormer timber frameworks, and external weathering membranes.
Our turnkey process handles full statutory compliance under the Party Wall etc. Act 1996, 30-minute protected fire enclosure routes under Approved Document B, and modern U-value insulation standards under Approved Document L.

What types of loft conversions do we build across London?

We shape each conversion around your roof’s structural profile, your family’s need for space, and whatever your local council’s planning rules allow, so the new room feels like it always belonged there rather than bolted on.

Dormer loft conversion — a structural extension that projects vertically out of an existing sloping roof, giving you square internal walls, standard ceiling heights, and room for proper vertical windows.

Hip-to-gable loft conversion — this replaces the sloping side roof (“hip”) of a semi-detached or end-terrace property with a vertical brick wall (“gable”), which opens up a lot more internal floor area.

Mansard loft conversion — a substantial roof restructuring with a near-vertical (72-degree) back wall and a flat top, giving you the most usable upper-floor volume of any option on period properties.

Velux rooflight conversion — an internal attic upgrade that fits high-performance rooflights flush into the existing roofline, with no change to the outside shape of the house.

Rear dormer and L-shaped dormer conversions

A rear dormer is still the most efficient and cost-effective way to add a master bedroom suite with an ensuite. For Victorian and Edwardian terraces with an existing rear outrigger — common right across Lewisham, Greenwich, Southwark, and Wandsworth — we build L-shaped dormers instead. That layout gives you two separate rooms on the top floor, typically a generous bedroom alongside a home office or family bathroom.

Hip-to-gable conversions for semi-detached houses

1930s semi-detached and end-of-terrace houses often have three sloping roof sides that eat into usable attic space. By pushing the sloping side ridge outward into a vertical gable wall, we free up the headroom needed for a proper central staircase, a decent-sized bedroom, and built-in eaves storage.

Mansard conversions for heritage and conservation settings

Mansard conversions tend to get a warmer reception from planning departments in Greater London’s conservation areas, since the 72-degree pitch sits comfortably alongside historic streetscapes. We finish them in matching slate, handmade brick slips, or zinc cladding, so the outside stays architecturally in keeping while the inside gets proper head height.

Velux and rooflight loft conversions

If your existing roof already has decent ridge height and you’d rather keep structural changes minimal, a Velux conversion is the quickest route to a genuinely usable room. We reinforce the existing floor joists, install high-efficiency insulation, and position rooflights to flood the attic with daylight, all without touching the roof profile. If you’re also after more space downstairs, this works well alongside our house extensions service, letting you tackle the whole property in one build.

How do loft conversion types compare for London homes?

The right style depends on your existing roof height, property type, how much Permitted Development allowance you have left, and your budget.

Conversion Style	Best Suited Property	Planning Permission Route	Primary Practical Benefit
Rear Dormer	Terraced & Semi-Detached	Permitted Development (up to 40m³/50m³)	Maximises head height and allows standard vertical windows
L-Shaped Dormer	Victorian & Edwardian Terraces	Permitted Development / Prior Approval	Creates two generous rooms (e.g. master suite + study)
Hip-to-Gable	1930s Semi-Detached & End-Terrace	Permitted Development (standard limits)	Converts restrictive side slope into full-height usable space
Mansard	Period Properties & Conservation Areas	Full Planning Permission required	Delivers maximum floor area with council-favoured aesthetics
Velux / Rooflight	High-Ridge Lofts (all property styles)	Permitted Development (Class C)	Lowest structural disruption and rapid build programme

Who is our loft conversion service designed for?

We handle the full design, statutory approvals, and precision construction for homeowners who want one team accountable for the whole job, with proper standards and careful project management throughout.

This service is ideal for:

Homeowners who’d rather have architectural plans, structural calculations, council certificates, and trade management sit with one experienced team.
Growing families in South East London and Greater London who need an extra bedroom, bathroom, or home office without stamp duty and the upheaval of moving.
Property owners dealing with party wall notices, shared chimney stacks, or strict conservation area rules.
Clients who want transparent milestone billing, a schedule they can rely on, and a 10-year structural warranty behind the finished work.

This service is not for:

Basic loft boarding or ladder installations meant purely for light storage, with no habitable reinforcement involved.
Self-managed projects where the homeowner plans to hire and coordinate carpenters, plumbers, and roofers independently.
Flats or maisonettes where the owner doesn’t hold the freehold or rights over the roof space.

How does our turnkey loft delivery framework work?

Converting a roof space calls for precise structural engineering, so loads transfer safely to the main load-bearing walls without putting unnecessary stress on the ceiling joists underneath. We keep structural design and site execution connected through one integrated process rather than passing the job between separate firms.

The Constructum Code Loft Delivery Framework

Measure internal head height and carry out a laser roof survey: we check your roof pitch, ridge height, chimney stacks, water tanks, and load-bearing walls before any concept design starts.
Engineer the structural steels and architectural layout: our in-house designers produce detailed floorplans and work out the exact RSJ steel beam specifications through our architectural design service.
Obtain Lawful Development Certificates and party wall sign-off: we manage council submissions, Building Control applications, and party wall surveyor notices through our planning approvals management team.
Put up scaffolding and complete the structural steel installation: we install heavy-duty scaffolding, hoist the engineered steel beams into the party walls, and build the new suspended timber floor joist system.
Frame, weather-seal, and insulate the dormer envelope: our carpenters frame the dormer or mansard structure, fit high-performance flat roof membranes, install double- or triple-glazed windows, and add rigid PIR insulation to meet Approved Document L standards.
Install the staircase, first-fix services, and fire protection: we fit the custom-made staircase over the existing flight, run certified plumbing and electrical circuits, and install FD30 fire doors with interlinked smoke alarms.
Finish the interior and issue your completion certificate: we plaster, fit sanitaryware, complete second-fix carpentry, and hand over your Building Regulations Completion Certificate alongside the 10-year structural warranty.

Quick answers to key technical questions

What is the minimum head height required for a legal loft conversion?

You want an existing ridge height of around 2.2 metres, measured from the top of the ceiling joists to the underside of the ridge timber, to comfortably achieve the 1.9-metre finished headroom at the centre of the stair that Building Regulations require, with 1.8 metres permitted at the outer edge.

Do I need planning permission for a rear dormer loft conversion?

Not necessarily — if the dormer stays within your Permitted Development volume limits (40m³ for terraces, 50m³ for semi-detached or detached houses) and uses matching materials, you can usually avoid full planning permission, as long as your property isn’t a flat or sitting in a conservation area with restricted permitted rights.

How do Building Regulations ensure fire safety in a three-storey converted house?

Approved Document B calls for a protected escape route with 30-minute fire-resistant doors (FD30) on every habitable room leading to the staircase, a fire-rated ceiling between floors, and mains-powered, interlinked smoke and heat alarms across all storeys.

What are the next steps to start your loft conversion?

We keep the consultation and pre-construction stages straightforward, so your project moves smoothly from that first enquiry through to site start.

Get in touch to talk through your project via our online contact portal or call us directly on +447802756785.
Book an on-site feasibility survey with our structural team to measure internal head height, assess the roof trusses, and review boundary walls.
Receive a detailed, line-item cost proposal and a full project programme covering every structural, architectural, and statutory phase.
Browse dormer, mansard, and hip-to-gable layouts in our portfolio of completed building projects to settle on your preferred fixtures and window arrangements.
Sign off on the project schedule and we’ll get started on the architectural drawings, structural calculations, and council submissions.

Frequently asked questions about loft conversions

How long does a standard loft conversion take to build in London?

A standard rear dormer or Velux loft conversion usually takes 6 to 10 weeks of active on-site work, while larger hip-to-gable or L-shaped dormers run to 8 to 12 weeks.

That splits into rough stages: the first 2 to 3 weeks cover external scaffolding, structural steel installation, and roof framing; weeks 4 to 6 involve external weatherproofing, window fitting, and staircase installation; and the final 3 to 4 weeks cover electrical wiring, plumbing, insulation, plastering, bathroom fitting, and second-fix carpentry. Before any of that starts on site, the design, structural calculations, and Lawful Development Certificate process usually take another 6 to 8 weeks.

Can we remain living in our house while the loft is being converted?

Yes — you can stay in your home through most of the build, since the bulk of the heavy structural work happens from the outside via scaffolding.

Our builders access the roof through an external opening, which keeps dust, materials, and heavy foot traffic out of your living space while the steel goes in and the frame goes up. The main disruption comes in the final weeks, when we open the ceiling to fit the new staircase, install fire doors, and connect plumbing and electrics — we put up dust screening to protect the rest of the house during that stage.

What happens if my existing loft ridge height is under 2.2 metres?

If your ridge height comes in below 2.2 metres, we can either lower the ceilings on the floor beneath using tie-beam hangers or, subject to planning approval, replace the roof structure with a higher ridge line.

Lowering the first-floor ceilings means fitting new floor joists suspended just above the existing ceiling level, which keeps the ground floor stable while securing the legal headroom needed upstairs. If your borough’s planning guidelines allow it, raising the roof ridge is the other option, and we’ll assess which route makes sense during the initial structural feasibility survey.

Do I need a Party Wall Agreement for a loft conversion?

You’ll need to serve a formal Party Wall Notice under the Party Wall etc. Act 1996 if your conversion involves inserting steel beams into a shared party wall or raising a boundary parapet wall.

Notice has to go to the adjoining owners at least two months before work starts. If your neighbours give written consent, the job proceeds under that notice; if they object, an agreed surveyor — or two separate ones — draws up a legally binding Party Wall Award covering working hours and protective measures, so construction can go ahead lawfully.

What warranties and completion certificates will I receive upon project handover?

You’ll get our formal 10-year structural warranty along with the official Building Regulations Completion Certificate issued by Building Control.

We also provide electrical installation certificates (Part P), Gas Safe certificates where new boilers or radiators are fitted, and manufacturer guarantees on roofing membranes and rooflights. That paperwork matters for home insurance and gives you certified proof of compliance whenever you come to sell.

Loft Conversions in South East and Greater London: Complete Design and Build Services