Loft Conversion Quotes London: How to Compare Costs and Choose the Right Builder
What a loft conversion quote should include, how London costs break down by conversion type, and how to compare three quotes side by side before you appoint anyone.
What are you renovating?
A loft conversion is one of the most reliable ways to add a bedroom and bathroom to a London house without losing garden or street space. It is also one of the easiest projects to get wrong at the quoting stage because the gap between a detailed, honest quote and a vague headline figure can be £20,000 or more, and the cheaper number is rarely what it appears.
This guide explains what drives loft conversion costs in London, what every quote should clearly state, and how to compare three quotes without being misled by the ones that look best on paper.
A Quote That Cost One London Homeowner £14,000 More Than It Needed To
In the autumn of 2024, the owners of a Victorian terrace on Tressillian Road SE4 in Brockley were planning an L-shaped dormer loft conversion. They received three quotes. The first, from a company they found through an online marketplace, came in at £54,500, a single figure on two pages with no breakdown of what was and was not included. The second, from a local contractor, came in at £68,000, itemised by trade, with structural engineer fees, building regulations, scaffolding, party wall notices, and decoration all separately listed. The third, from a specialist loft company, was £72,000 with a similar level of detail.
They chose the £54,500 quote. By the time the conversion was complete, the final bill was £68,400, almost exactly what the itemised quote had originally said. The difference came from structural engineer fees the original quote excluded, a steelwork upgrade required by building control, and redecoration not mentioned in the initial scope.
The project was not unusual. The quote was.
What it is: The simplest form of loft conversion, with no structural changes to the roof shape. Velux or similar rooflights are cut into the existing roof slope, and the loft floor is strengthened and insulated. A staircase is added.
Best for: Properties with sufficient existing head height (typically 2.2m minimum at the ridge) and where planning or budget restricts structural works. Common in outer London boroughs where rooflines are less constrained.
What you get: A usable room, often a study, guest bedroom, or single bedroom within the existing roof envelope. No external change to the roofline.
Planning: Permitted Development in most cases, provided rooflights do not protrude above the roof slope and face the street.
London cost range: £20,000–£40,000
Dormer Loft Conversion
What it is: A box-shaped extension to the existing roof, usually at the rear, that creates a vertical rear wall and a flat or low-pitch roof above. The most common loft conversion type in London.
Best for: Victorian and Edwardian terraces and semis across inner London where head height is insufficient for a rooflight-only conversion. The rear dormer can run the full width of the house or a partial width.
What you get: A full-height room at the rear of the loft, typically a double bedroom with an en-suite bathroom. The flat-roof dormer is utilitarian; a pitched or zinc-clad dormer is more architecturally considered and may be required in conservation areas.
Planning: Rear dormers are Permitted Development on most houses (not flats), subject to size limits; the dormer must not exceed the height of the existing roof ridge, must be set back from the eaves, and must not face the highway. Conservation area designation removes PD rights for rear dormers facing a highway and for all side dormers.
London cost range: £45,000-£85,000
L-Shaped Dormer Loft Conversion
What it is: A dormer that extends across both the main rear roof and the roof of the back addition, the lower single-storey or two-storey rear extension common on Victorian and Edwardian terraces. The resulting L-shape creates significantly more floor area than a standard dormer.
Best for: Victorian mid-terraces with a back addition, particularly common across inner south and east London: Lewisham, Southwark, Hackney, Lambeth. The back addition roof is typically lower than the main roof, and filling both with dormers creates an L-shaped loft room at two levels.
What you get: Two connected loft rooms, often a master bedroom with en-suite on the main roof section and a smaller room, dressing area, or bathroom on the back addition. The L-shape is the most space-efficient loft conversion available on a Victorian terrace.
Planning: As per the rear dormer, typically PD, subject to limits. The back addition dormer faces the rear and is usually straightforward. Conservation area restrictions apply as above.
London cost range: £60,000–£95,000
Hip-to-Gable Loft Conversion
What it is: Converts the sloping hip end of a roof into a vertical gable wall, extending the ridge and creating a full-width loft room. Usually combined with a rear dormer.
Best for: End-of-terrace houses and semi-detached houses where the hip end faces a side boundary. Very common in outer London’s interwar semis in Ealing, Merton, Barnet, Bromley.
What you get: A substantially larger loft room than a dormer alone would achieve; the new gable wall replaces the sloping hip and gives full head height across the full width of the loft. Combined with a rear dormer, a hip-to-gable conversion can accommodate two bedrooms and a bathroom.
Planning: Hip-to-gable conversions change the external appearance of the roof and almost always require full planning permission, as they fall outside PD limits (which do not allow extensions that increase the height of the ridge on a semi or end-terrace beyond specific conditions). Some local authorities are more permissive than others; a pre-application enquiry is worthwhile.
London cost range: £55,000–£90,000
Mansard Loft Conversion
What it is: Replaces the upper portion of the roof with near-vertical rear (and sometimes front) walls and a shallow pitched roof, creating maximum internal volume. The most expensive and most regulated loft conversion type.
Best for: Properties in central and inner London conservation areas where a standard dormer would be refused but a mansard, which reads as a roof alteration rather than an extension, may be acceptable. Common on Victorian terraces in Kensington, Islington, Camden, and Hammersmith.
What you get: The largest internal volume of any loft conversion type. A mansard can accommodate a generous master bedroom, dressing room, and en-suite within a conversion that reads as part of the original roofscape rather than an addition.
Planning: Always requires full planning permission. Mansard designs are subject to tight design guidance in conservation areas; materials, pitch angle, set-back from the parapet, and fenestration are all controlled. An experienced conservation architect is essential.
London cost range: £75,000–£120,000
What Should a Loft Conversion Quote Include?
A loft conversion quote that gives you a single figure without breaking it down is not a quote you can compare. Before you can assess whether you are paying fairly, the following must be stated separately:
Structural works: Loft floor strengthening (or new floor joists), steel beams where required, and any changes to the existing roof structure. These are often the largest variable; a mansard or hip-to-gable involves substantially more structural work than a rooflight conversion.
Carpentry and roofing: Dormer framing, new rafters, waterproofing membrane, and roof covering (lead, zinc, EPDM rubber, or tiles). The material specified affects both the cost and the planning acceptability.
Insulation: The type and depth of insulation must meet Building Regulations Part L. Cold roof, warm roof, and hybrid approaches have different costs. A quote that does not specify insulation type is leaving a gap.
Windows and glazing: Rooflights, dormer windows, and any Juliet balconies. Specify brand, opening configuration, and glazing specification. Conservation area projects may require timber frames rather than uPVC.
Staircase: The staircase is a significant cost item and must be sized and positioned to meet Building Regulations for a habitable room (not a fixed ladder or space-saver stair). Confirm whether it is designed, supplied, and fitted within the quote.
Plumbing and electrics: If the conversion includes an en-suite, confirm that a plumber is named and included. Electrical first and second fix for the new rooms’ sockets, lighting, and smoke detection must be explicitly covered, or the quote will generate extras.
Building Regulations fees: A full plans application costs around £400–£700 for a residential loft conversion. Confirm whether this is included or in addition.
Structural engineer: The engineer’s fee for calculating beam sizes and floor loadings is typically £600–£1,500. It is frequently excluded from loft conversion quotes; confirm explicitly.
Scaffolding: External scaffold for a rear dormer is typically £1,500–£3,500. Its inclusion or exclusion must be stated.
Party wall: If the loft conversion involves works to a shared party wall (common on terraces), party wall surveyor fees are typically £800–£2,500 per adjoining owner. These are rarely included in a contractor quote and must be budgeted separately.
Plastering, decoration, and flooring: Finishing works vary widely. Confirm whether the quote includes plasterboarding and skimming, and whether decoration and flooring are included or excluded.
Waste removal: Skip hire, scaffold dismantling waste, and general clearance.
| Conversion Type | Best For | Typical Size Gained | London Cost Range | Planning |
|---|---|---|---|---|
| Velux / Rooflight | Properties with adequate existing head height | 15–25 m² | £20,000–£40,000 | Usually PD |
| Rear dormer | Victorian & Edwardian terraces and semis | 20–35 m² | £45,000–£85,000 | PD or Planning |
| L-shaped dormer | Victorian terraces with back addition | 30–50 m² | £60,000–£95,000 | PD or Planning |
| Hip-to-gable | End-of-terrace and semi-detached houses | 25–40 m² | £55,000–£90,000 | Planning required |
| Mansard | Conservation areas in central & inner London | 35–55 m² | £75,000–£120,000 | Planning required |
| Full floor addition | Flat-roofed properties or large semis seeking maximum space | 50–80 m² | £100,000–£180,000 | Planning required |
London all-in ranges including structure, finishes, building regulations, and VAT. Excludes structural engineer fees (typically £600–£1,500), party wall surveyor costs (£800–£2,500 per adjoining owner), architect fees, and furniture. Add 10–15% contingency.
Why Are Loft Conversion Quotes in London So Different From Each Other?
Three quotes for the same loft conversion on the same street can differ by £15,000 to £25,000. The reasons are rarely about one contractor being better value than another. They are about scope.
Different structural assumptions
Until a structural engineer assesses the existing floor and roof, the beam sizes and floor loading upgrades cannot be confirmed. Contractors who quote before an engineer’s report is available are estimating; those who have seen an engineer’s output are pricing. The estimate tends to be lower and revised upward once work begins.
Specialist vs generalist labour rates
A loft conversion specialist who does twenty conversions a year in inner London will price scaffolding, roofing, and dormer framing accurately because they have done it recently. A general builder doing their second conversion may underestimate the same elements.
Exclusions buried in small print
Structural engineer fees, scaffolding, building regulations applications, party wall notices, and decoration are regularly excluded without being flagged. A quote that lists exclusions clearly is more useful than one that does not, even if the headline figure is higher.
Conservation area uplifts
A conversion in a conservation area requiring a timber-framed mansard with lead cladding costs substantially more than the same footprint with a standard EPDM flat-roof dormer. If the quote does not reflect conservation area requirements, it is not priced for your project.
What Are the Red Flags in a Loft Conversion Quote?
- A quote under five pages with no trade breakdown
- No mention of structural engineer fees or building regulations
- Payment schedule requiring more than 20–25% upfront
- No validity period; prices can change, and an open-ended quote is meaningless
- “Subject to survey” without a follow-up survey before you sign
- Scaffolding listed as a provisional sum without a figure
- No reference to party wall obligations on a terrace property
- References only from outside London; period property conversions in London have specific requirements
How to Find the Right Loft Conversion Builder in London
The right loft conversion builder for a Victorian terrace in Hackney is not necessarily the right builder for a mansard in Kensington or a hip-to-gable semi in Ealing. Each conversion type has its own structural, planning, and material requirements, and recent local experience is the best predictor of whether a project will run to budget and time.
Before appointing any contractor:
- Ask for two or three references from conversions of the same type in London within the last 24 months, and call them
- Confirm public liability insurance of at least £2 million and check the certificate, not just a verbal assurance
- Ask who will manage the structural engineer, building regulations, and party wall process and whether those fees are included or additional
- Request a payment schedule tied to completed stages, not a large upfront sum
QuoteMyRenovation vets every contractor on the platform against their insurance, trade qualifications, and recent project history before they receive leads. When you submit your loft conversion brief, you receive up to three matched quotes from builders with verifiable experience in your borough and conversion type, compared on the same brief.
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