Cost Guide · 2026
How Much Does a Loft Conversion Cost in 2026?
Real 2026 price ranges for every loft conversion type — Velux, dormer, hip-to-gable, L-shape, and Mansard — across London and the South-East. Written by RCB, a principal contractor who builds them every week.
Price Ranges
Loft conversion costs by type — 2026
The biggest factor in loft conversion cost is the type of conversion you choose. Below are honest 2026 price ranges for Greater London and the South-East, based on real project data. These assume a single bedroom, en-suite bathroom, and a new staircase as standard scope.
| Conversion Type | Typical Cost Range |
|---|---|
| Velux / Rooflight | £25,000 – £35,000 |
| Rear Dormer | £35,000 – £55,000 |
| Hip-to-Gable | £45,000 – £60,000 |
| L-Shape Dormer | £55,000 – £75,000 |
| Mansard | £60,000 – £80,000 |
These are ballpark figures only, provided for high-level budgeting. Not a formal quotation. Actual costs vary depending on design, compliance, procurement, and site complexities. A detailed estimate requires a site visit and full drawings.
Cost Drivers
What affects the cost of a loft conversion?
Conversion type and roof structure
The roof type on your property dictates what is structurally possible and how much steelwork is required. Mansards and hip-to-gable conversions require the most structural intervention. Velux conversions require the least. The more you alter the roof envelope, the higher the cost.
Floor area and headroom
Larger loft spaces cost more to convert — more joists to replace, more insulation, more boarding, more plastering. Equally important is existing headroom: a ridge height below 2.2m makes a usable conversion difficult and may require raising the roof structure, adding significant cost.
Staircase design and position
A well-positioned staircase that lands in a hallway or landing is straightforward. One that requires re-configuring a bedroom, moving a soil stack, or taking floor area from a living space adds design complexity and cost. Bespoke joinery staircases also cost significantly more than standard softwood.
Bathroom and plumbing
Adding an en-suite means routing soil pipes, hot water, and cold water supply up through the property. The further the new bathroom is from your existing soil stack and boiler, the more complex — and expensive — the plumbing becomes.
Windows and natural light
Standard Velux roof windows are cost-effective. Bespoke dormers with aluminium-framed sliding or casement windows, Juliet balconies, or structural glazing cost significantly more. Premium glazing can add £5,000–£15,000 to a project depending on specification.
Planning status and location
Properties in conservation areas, Article 4 direction zones, or listed buildings face additional planning requirements. A Mansard in a conservation area may require a full planning application, pre-application advice, and heritage design statements — adding £5,000–£12,000 in professional fees before a single brick is laid.
What You Get
What's included in the price
Not Included
What's NOT included
RCB does not include client-supplied finishes in the headline build cost. These are items where your personal choice drives the price — quoting them in the build price would only give you a generic standard you may not want.
Budget Planning
Additional costs to budget for
The build cost is only part of the total investment. A realistic whole-project budget for a loft conversion also includes:
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FAQs
Common questions about loft conversion costs.
How much does a loft conversion cost in 2026?
A loft conversion in London and the South-East costs between £25,000 for a basic Velux conversion and £80,000+ for a full Mansard conversion. The most common type — a rear dormer with bedroom and en-suite — typically costs £35,000–£55,000.
What is the cheapest type of loft conversion?
A Velux (rooflight) conversion is the most affordable at £25,000–£35,000 because it does not change the roof structure. It only works where existing headroom is already sufficient — typically 2.2m or more from floor to ridge.
Do I need planning permission for a loft conversion?
Most rear dormer and Velux loft conversions fall under Permitted Development and do not require a planning application, provided they meet size limits and do not alter the front roofline. Mansard conversions and properties in conservation areas usually require full planning permission. Building Regulations approval is always required.
How long does a loft conversion take?
The build itself typically takes 8–14 weeks depending on type and complexity. Velux and simple dormers are at the lower end; mansards and L-shaped dormers towards the higher end. Pre-construction (drawings, planning, Building Regulations) adds 8–20 weeks.
What adds value to a loft conversion?
A loft conversion with a bedroom and en-suite typically adds 15–25% to a property's value in London and the South-East. The addition of a dedicated staircase, storage, and good natural light through large windows or dormers maximises the return.
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