Loft Conversion Roof Guide: Types, Costs and Planning (2026)
A loft conversion adds 15 to 20 percent to your property value at roughly half the cost per square metre of a ground-floor extension. But the type of conversion you can do depends almost entirely on your existing roof — its structure, pitch, height and what planning rules allow in your area. Get the roofing element wrong and you end up with inadequate headroom, condensation problems or a costly enforcement notice.
This guide focuses specifically on the roof considerations: structural changes, conversion types, insulation requirements and costs. For general planning rules applying to all roof work, see our planning permission guide.
Types of Loft Conversion: Which Suits Your Roof
There are four main conversion types. Which ones are viable depends on your roof's pitch, ridge height and structure — not just what you prefer aesthetically.
| Type | Typical cost | Planning | Best suited to |
|---|---|---|---|
| Velux (roof light) | £25,000 to £35,000 | Permitted development (usually) | Roofs with 2.2m+ headroom already |
| Dormer | £30,000 to £50,000 | Usually permitted development (rear) | Most semi-detached and terraced houses |
| Hip-to-gable | £35,000 to £55,000 | Often needs planning permission | End-of-terrace and detached properties |
| Mansard | £45,000 to £70,000 | Almost always needs planning permission | All types, maximises space |
Velux (Roof Light) Conversion
The simplest and least expensive option. Roof windows are fitted into the existing slope without changing the roof's shape. The loft space is converted using the existing structure.
Requires: A minimum of 2.2m headroom measured from the top of the existing ceiling joists to the underside of the ridge. Less than this and most adults cannot stand comfortably in any part of the space — the conversion exists on paper but is impractical.
Structural work: Ceiling joists are upgraded to take floor loading. Steel beams may be required depending on span and load. Roof structure is generally not altered.
Planning: Usually permitted development. Roof light windows that project more than 150mm from the roof slope or sit higher than the ridge require planning permission. Conservation area and listed building restrictions apply.
Insulation: The most common approach is warm roof insulation between and below the rafters, using PIR boards (Celotex, Kingspan) to achieve the required U-value (0.18 W/m2K under Part L). This is a more complex installation than cold loft insulation and affects the ceiling depth in the converted space.
Best for: Properties with naturally high roof pitches and existing headroom. Often the only viable conversion on properties with Article 4 Directions or in conservation areas.
Dormer Conversion
An extension built out from the rear roof slope, creating a flat-roofed or pitched-roof box that adds vertical headroom and significantly more usable floor area than a Velux-only conversion. This is the most common conversion type in the UK.
Structural work: The original roof structure is altered — rafters are cut and trimmed, steel beams (typically RSJs or box beams) carry the new loads, and the dormer structure is built with its own flat or pitched roof. The party walls (in a semi) may be used as structural support, which may trigger Party Wall Act requirements.
Planning: Rear dormers on most houses are permitted development provided they:
- Do not add more than 50 cubic metres of additional roof space (40 cubic metres for terraced houses)
- Are not higher than the existing roof
- Use materials that match the existing house as far as practicable
- Are set back at least 200mm from the eaves
- Do not face a highway
Side dormers and front dormers almost always need planning permission. Conservation areas may restrict rear dormers too.
Roof on the dormer itself: Usually flat (felt, EPDM or GRP) or low-pitched with lead or standing seam metal. The flat roof element of a dormer requires exactly the same material consideration as any other flat roof — a 15-year felt dormer roof on a 50-year conversion is a false economy.
Best for: Standard semi-detached and terraced properties where rear permitted development rights are available. Provides a practical bedroom and en-suite in most cases.
Hip-to-Gable Conversion
The sloping hip end of the roof is extended outward to create a vertical gable wall. This adds significant floor area compared to a Velux conversion and is often combined with a rear dormer for maximum space.
Who it suits: End-of-terrace houses with hipped (sloping) ends. Detached properties with four-sided roofs. Not applicable to properties that already have gable ends (most mid-terrace houses).
Structural work: The hip rafters are removed and a new gable wall is built in brick, block or timber frame. This is more significant structural work than a standard dormer. The new wall needs foundations at the correct depth and a lintel over any new window openings.
Planning: Hip-to-gable conversions frequently require planning permission because they change the roof profile significantly. Permitted development allowed for some hip-to-gable extensions exists but is subject to restrictions — check your specific property with the local planning authority rather than assuming. Conservation area and Article 4 restrictions almost always exclude hip-to-gable conversions from permitted development.
Party wall considerations: On an end-of-terrace or semi-detached property, the hip end may be adjacent to a neighbour's structure. Even where it is not directly a party wall, any excavation or building near the boundary may trigger Party Wall Act requirements.
Mansard Conversion
A mansard conversion rebuilds the rear (and sometimes front) roof slope at a near-vertical angle (typically 70 to 72 degrees) with a flat roof on top. It creates the maximum possible usable space and allows full-height rooms beneath the new slope.
Cost: £45,000 to £70,000 and upward for a standard residential mansard. This reflects the extent of roof rebuilding involved — it is substantially more work than other conversion types.
Planning: Almost always requires full planning permission because it significantly changes the roof profile. In many areas, planning departments view mansards as a higher-impact change and scrutinise them more closely on design quality. Mansards are common in London (particularly Victorian terraces) where planners have well-established design guidance. Outside London, acceptance varies by local authority.
Structural implications: The existing rear slope is entirely rebuilt. This is a major structural project involving temporary propping of the existing structure during work. A structural engineer should be involved from the design stage — budget approximately £1,000 to £2,000 for structural engineering as part of the total project.
Building regulations: Full Part A (structure), Part B (fire), Part F (ventilation), Part L (energy) and Part K (stairs and guarding) requirements apply. A mansard in a terrace also typically triggers Part E (sound insulation) checks at party walls.
Insulation and Building Regulations for Loft Conversions
All loft conversions require full building regulations approval — there is no self-certification or permitted development exemption for conversion work. The key requirements relevant to the roof:
Part L (Energy Efficiency): The converted roof area must achieve a U-value of 0.18 W/m2K maximum. For most rafter depths, this requires PIR (polyisocyanurate) insulation between and below rafters. The specific thickness depends on the rafter depth and the insulation's lambda value — a structural engineer or your architect can calculate this.
Ventilation gap: If using a breathable (vapour-permeable) underlay, a 25mm ventilation gap is still recommended above the insulation to prevent interstitial condensation. With older impermeable underlays, the ventilation gap between insulation and underlay is mandatory. Getting this wrong leads to condensation and timber rot within a few years of conversion.
Part B (Fire Safety): Loft conversions must provide a protected escape route from the new storey — typically a fire door at the head of the stairs and fire-rated construction around the stairwell. Mains-powered smoke alarms are required on each floor.
Staircase (Part K): The staircase serving the loft must meet minimum width (minimum 600mm) and headroom (2.0m at the centre, 1.8m at the sides) requirements. This often constrains the design more than the structural elements.
Frequently asked questions
How much does a loft conversion add to house value?
Typically 15 to 20 percent of property value when a genuine bedroom and bathroom are added. The actual figure depends on the local market, the quality of the conversion and the existing property. A £400,000 house in a market where four-bedroom properties sell at £500,000 may see a clear £100,000 uplift. In markets where four-bedroom homes do not command a strong premium, the return is lower.
How do I know if my loft is suitable for conversion?
The key measurement is the height from the top of the ceiling joists to the underside of the ridge. A minimum of 2.2m is recommended for any practical conversion. Below 2.2m, a Velux-only conversion produces a very cramped space. A dormer or mansard can add headroom where the natural pitch is inadequate, but at higher cost. A structural engineer or specialist loft conversion company can assess your roof in 30 minutes.
Do I need an architect for a loft conversion?
You need someone to produce detailed drawings for building regulations approval. This can be an architect, an architectural technician or a specialist loft conversion company with in-house design. For planning applications, a qualified architect is usually worthwhile — they understand what planners want to see and can produce plans that minimise revision requests. For a straightforward dormer on a standard house, a design and build loft conversion company can often manage planning, design and build in one package.
Can I live in my house during a loft conversion?
Usually yes, though it is disruptive. The main disruption phase is when the roof structure is being altered — temporary weather proofing is required and there will be noise, dust and limited access to parts of the house. Most loft conversions complete in six to ten weeks. The roof-open period (where temporary sheeting covers the work area) is typically two to four days for a standard dormer.
Is spray foam insulation a problem in a loft conversion?
Yes. Spray foam insulation applied to the underside of roof tiles by a previous owner causes significant problems. It bonds tiles together and to the battens, making tile-by-tile inspection and repair impossible without removing and destroying the foam. Many mortgage lenders now refuse lending on properties with spray foam in the roof space until it is removed and the roof is inspected. If your loft has spray foam, get specialist advice before planning a conversion — removal costs typically run £1,000 to £3,000 and the roof timbers must be inspected after removal.