TL;DR: Loft conversion costs in the UK vary by type. A simple Velux (rooflight) conversion in an existing usable loft is the cheapest option. A dormer conversion (the most common) sits in the middle. A hip-to-gable or mansard conversion typically costs the most because it changes the roof structure substantially. Beyond the build cost, owners need to budget for VAT (20 percent on most residential building work), structural and architectural fees, building control, party wall agreements where they apply, and finishes such as flooring and bathroom fittings. Most loft conversions on standard houses are permitted development under the General Permitted Development Order, but conditions apply, and building regulations approval is always required regardless of planning route.
Last reviewed May 2026
A loft conversion is one of the most common and cost-effective ways to add usable space to a UK house. It avoids the planning complications of an extension that takes up garden space and, in many cases, falls within permitted development rights. The cost varies widely depending on the type of conversion, the condition of the existing loft, the location, the contractor, and the level of finish.
The figures quoted in industry surveys and trade body publications give a useful range, but every job is priced individually. The headline cost is only one part of the budget. Professional fees, building control charges, VAT, party wall agreements where they apply, and the cost of fitting out the new space (flooring, bathroom, electrics, decoration) all need to be included to get a realistic total.
This guide sets out the main types of loft conversion, what drives the cost of each, the additional charges to expect, and the planning and building regulations framework that applies.
The four main types of loft conversion
Loft conversions fall into four main categories, listed from cheapest to most expensive in terms of typical cost.
A Velux or rooflight conversion is the simplest. It uses the existing roof structure and adds rooflights to provide natural light. There is no change to the roof shape from the outside. It is suitable only where the existing loft already has enough head height (typically at least 2.2 to 2.4 metres at the highest point) and a useable footprint without further structural alteration. Building regulations still apply to flooring, insulation, escape and ventilation.
A dormer conversion adds a box-shaped or pitched extension out from the existing roof slope to create extra floor space and head height. Dormer conversions are by far the most common type in suburban housing, particularly for terraced and semi-detached properties at the rear. The cost is higher than a Velux because the roof structure is opened up and new external walls and roof are built.
A hip-to-gable conversion changes the side hipped roof of a house (typical on semi-detached and end-of-terrace properties) into a vertical gable wall. This significantly increases the volume of usable space inside. It is often combined with a rear dormer for maximum effect. The work is more substantial than a simple dormer and the cost reflects that.
A mansard conversion replaces the existing roof slope with a much steeper, almost-vertical structure, creating an additional storey under a new shallow-pitched flat roof. Mansard conversions provide the largest gain in usable floor area and head height but require the most extensive structural work and are often subject to tighter planning controls, particularly in conservation areas.
What drives the headline cost
Within each category, the actual quote depends on a number of factors. The size of the existing loft is the obvious one: a larger conversion costs more in materials and labour. Roof structure (traditional rafters versus modern trussed roofs) affects how easily the existing structure can be modified; trussed roofs typically require more work because the trusses cannot simply be cut without compensating structural changes.
The position of the staircase matters. Building regulations require that the new staircase must comply with rules on rise, going, headroom and protected route for escape from fire. Fitting a code-compliant staircase into a constrained space sometimes requires sacrificing room downstairs (for example, removing a chimney breast or reconfiguring a landing).
The level of finish also affects cost. A bedroom-only conversion is cheaper than a bedroom-plus-en-suite, which is in turn cheaper than a self-contained suite with a high-end bathroom and bespoke joinery. Adding a bathroom involves drainage from upper floors, which can be straightforward or complex depending on the layout.
Location matters because labour rates vary regionally. London and the South East typically attract higher rates than the rest of the UK. The premium can be 20 to 30 percent or more for the same job specification.
Costs beyond the headline build figure
The build quote is normally just the labour, materials and contractor's overheads. Several other items have to be added.
VAT applies at 20 percent on most residential building work in the UK, including loft conversions. Some specific energy-saving works can attract reduced or zero VAT under HMRC rules, but the bulk of a loft conversion is standard-rated.
Architectural and structural engineering fees cover the design drawings and structural calculations needed for the build and for building control approval. Architects typically charge a percentage of the build cost or a fixed fee. Structural engineer fees are usually a separate fixed amount.
Building control fees are paid to the local authority (or to an approved private building control inspector) to inspect and certify that the work meets the Building Regulations. The fee depends on the work value and the local authority. The certificate is essential evidence for a future house sale.
Party wall agreements may be required under the Party Wall etc. Act 1996 where the work affects a shared wall (typical in terraced and semi-detached properties). Obtaining a party wall award through a surveyor is a separate fee, paid by the building owner. Where the neighbour appoints their own surveyor, the building owner usually pays both.
Planning permission and permitted development
Most loft conversions on standard houses in England can be carried out under permitted development rights under Class B of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015. The conditions include limits on the additional roof volume (40 cubic metres for terraced houses and 50 cubic metres for detached and semi-detached), on materials matching, on extensions beyond the principal elevation, and on the property not being in a designated area such as a conservation area or Area of Outstanding Natural Beauty.
Where the work falls outside permitted development (a mansard, an unusually large dormer, a property in a designated area, or a flat) full planning permission is required. The planning application fee, the work involved in preparing the application and the time required to obtain permission all add to the project's overall cost and timeline.
Permitted development is not the same as not needing approval at all. A homeowner can apply for a Lawful Development Certificate from the local planning authority to confirm that a particular project falls within permitted development. This is useful evidence for a future sale and avoids any later dispute. The application has its own fee.
Building regulations approval is always required for a loft conversion, regardless of planning route. The regulations cover structural integrity, fire safety, insulation, ventilation, sound and the staircase. Building control sign-off is essential for safety and for resale.
Timeline and what it means for occupants
A typical residential loft conversion takes several weeks to a few months on site, depending on the scope. A simple Velux conversion might be completed in 4 to 6 weeks. A dormer or hip-to-gable can take 8 to 12 weeks. A mansard is generally longer.
The early weeks involve external work (roof opened, new structure built, weatherproofing) which can be disruptive in terms of noise, scaffolding and dust but generally does not require the household to move out. The later stages are internal fit-out and decoration. Most households continue to live in the property throughout the build.
Lead times also need to be added. Reputable contractors are often booked months ahead, and architectural design, structural calculations and (where needed) planning permission can take additional weeks before the build can start. The full project from initial contact to finished room can comfortably run six months or longer.
Value added to the property
Industry research and surveyor data from organisations such as Nationwide and the Royal Institution of Chartered Surveyors (RICS) consistently show that a well-executed loft conversion adds value to a property, particularly where it adds an extra bedroom (and especially an en-suite). Estimates of the percentage uplift vary, but the broad finding is that loft conversions are one of the more cost-effective ways to add value compared with the build cost.
The value uplift depends on local market conditions. In areas where buyers are looking for additional bedrooms and where the local "ceiling" price has not yet been reached, the uplift can comfortably cover the cost. In areas where prices are already at or near the ceiling for the type of property, the financial gain is smaller. Local estate agents and recent sold prices for similar properties give the most accurate guide.
For homeowners not motivated primarily by resale value, the practical benefit of additional usable space (an extra bedroom, a home office, a play room) is often the more important consideration. A loft conversion typically delivers more space per pound spent than a side or rear extension.
Disclaimer: This article is general information about the cost and process of loft conversions in the UK. It is not professional building, planning, surveying or legal advice. Costs vary by region, property type and specification, and the planning and building regulations position depends on the specific property and proposal. Anyone planning a loft conversion should obtain detailed quotes from reputable contractors, take advice from an architect or structural engineer, and check the planning and building regulations position with the local planning authority.
Frequently asked questions
What type of loft conversion is the cheapest?
A Velux or rooflight conversion is generally the cheapest because it uses the existing roof structure without external alteration. It is only suitable where the existing loft has enough head height and floor area to be useable. Where it is feasible, it can be a fraction of the cost of a dormer or mansard conversion.
Do I need planning permission for a loft conversion?
Many loft conversions on standard houses in England fall within permitted development rights, subject to volume and design conditions. Properties in conservation areas, AONBs and other designated areas, and proposals that exceed permitted development limits, require full planning permission. Building regulations approval is always required, regardless of planning route.
How long does a loft conversion take?
Typical site times are 4 to 6 weeks for a Velux conversion, 8 to 12 weeks for a dormer or hip-to-gable, and longer for a mansard. Add several months for design, planning (if required) and contractor lead time. The full project from initial contact to finished room often runs six months or more.
Will the work disrupt my household?
Most households remain in the property throughout the build. The early weeks (roof opened, new structure built) are the most disruptive in terms of noise, dust and scaffolding. Internal fit-out is generally less intrusive. A reputable contractor should set out a programme showing what to expect at each stage.
Does a loft conversion add value to the property?
A well-executed loft conversion typically adds value, particularly where it provides an additional bedroom or an en-suite, and is widely regarded as one of the more cost-effective ways to add space and value compared with the build cost. The actual uplift depends on local market conditions and the property's existing position relative to local ceiling prices.
How we verified this
The categories of loft conversion, the planning permission position and the role of permitted development reflect the Town and Country Planning (General Permitted Development) (England) Order 2015 and current Planning Portal guidance. Building regulations requirements for loft conversions are covered in the Building Regulations 2010, principally Approved Documents A (structure), B (fire safety), K (protection from falling), L (conservation of fuel and power) and Part M. The Party Wall etc. Act 1996 governs party wall arrangements. VAT treatment of residential building work reflects HMRC's published guidance on VAT for the construction industry. Cost ranges referenced are descriptive and vary substantially by location, type of conversion and specification; site-specific quotes from reputable contractors are the only reliable source of pricing for a particular project.
