Types of Dormer Windows for Loft Conversions: Which Is Right for Your London Property?

**What a dormer window does**:

• A dormer window is a structural element that projects vertically from the slope of the existing roof, creating a section of vertical wall with a window in it — and in most cases, a flat or pitched roof over the dormer itself. The purpose of a dormer is to:
• •Create full head height (typically 2.3m–2.5m) over a larger portion of the loft floor area than would otherwise be achieved with the existing sloping roof
• •Provide natural light and ventilation to the loft room
• •In some cases, provide an emergency escape route

Without dormers, a loft conversion is limited to the 'Velux' or roof window conversion type — where Velux-type rooflights are installed flush with the existing roof slope but no structural roof extension is made. A Velux-only conversion is the most restricted type in terms of usable floor area (head height of 2.0m+ is only achievable at the ridge and falls quickly on either side), but it is also the simplest, cheapest, and most likely to be within permitted development rights.

**Type 1 — Box dormer (also called a flat-roof dormer)**:

A box dormer is a rectangular dormer window with a flat roof (usually single-ply membrane such as EPDM or GRP — see `flat-roof-extension-guide`). The vertical cheek walls of the dormer are clad (typically zinc, slate, tile hanging, or timber boarding). The box dormer is the most common and most cost-effective dormer type for rear-facing London loft conversions.

*What it achieves*: A full-width rear box dormer (running from party wall to party wall) creates a flat-ceilinged loft room approximately 2.3m–2.5m in height across the full width of the property's rear elevation. This is the type that maximises the habitable floor area of the loft. A single box dormer (narrower, in the centre of the rear slope, not full-width) creates less space but may be more acceptable in certain planning contexts.

*Planning status*: Full-width rear box dormers on a standard London Victorian terrace house are typically within permitted development (PD) rights — they can be built without planning permission provided they meet the PD size limits (do not exceed the highest part of the original roof, do not extend beyond the rear wall of the original house, have materials similar in appearance to the existing house). However, box dormers at the front or side of the house require planning permission. In conservation areas, many councils require planning permission for any dormer — including rear dormers.

*Typical cost*: £8,000–£18,000 for the dormer structure itself (within a loft conversion project).

**Type 2 — Shed dormer (also called a full-width or full-slope dormer)**:

A shed dormer has a very low-pitched or nearly flat roof that replaces a large section of the existing rear roof slope — effectively 'pushing back' the rear slope to create a large section of near-vertical wall. The effect is similar to a large box dormer but with a lower-pitch roof over the dormer section rather than a fully flat roof.

*What it achieves*: Very similar to a full-width box dormer — creates good head height across the full width of the rear. The shed dormer roof profile (low-pitch) is sometimes preferred visually over the flat-roofed box dormer.

*Planning status*: As per box dormers — typically PD at the rear, requires planning at the front, planning permission in conservation areas.

*Typical cost*: Similar to a box dormer — £8,000–£16,000 for the structure.

**Type 3 — Hip-to-gable conversion**:

Many London semi-detached houses and some end-of-terrace houses have a 'hipped' roof — where the roof slopes inward on the side (flank) elevation rather than ending in a gable. Converting the hip to a gable (replacing the inward-sloping hip with a vertical gable wall) creates significant additional loft volume on the side of the property.

*What it achieves*: A hip-to-gable conversion extends the full-head-height area of the loft all the way to the side flank wall — eliminating the restricted head height that the hipped section would create. Combined with a rear box or shed dormer, a hip-to-gable + rear dormer conversion (the 'L-shaped' conversion in marketing terms) creates a very large loft room.

*Planning status*: Hip-to-gable conversions on semi-detached houses are typically within permitted development (the PD rules allow extension up to the ridge height without planning permission for the gable wall). However, the new gable wall faces toward the side (and often toward the neighbour's garden) — some local authorities require planning in certain circumstances, and in conservation areas, planning permission is typically required.

*Typical cost*: £15,000–£25,000 for the hip-to-gable alteration, in addition to the cost of a rear dormer and the rest of the loft conversion.

**Type 4 — Mansard dormer**:

A mansard dormer replaces the entire rear roof slope with a near-vertical mansard slope (typically 70° to the horizontal — almost vertical but not quite) with a flat roof behind. The term 'mansard' comes from the 17th-century French architect François Mansart. The mansard dormer creates the maximum possible loft volume of any dormer type — the entire rear slope becomes habitable space.

*What it achieves*: Near-full-height space (2.3m–2.5m) across the entire width and most of the depth of the loft. The visual effect from outside is of an additional floor on the rear of the house with a steeply sloped face. The mansard conversion is the most expensive loft conversion type but also the largest space generator.

*Planning status*: Mansard conversions are NOT within permitted development — the near-vertical slope and the flat roof of a mansard do not meet the PD criteria (which require the dormer not to exceed the plane of the original roof). A mansard conversion always requires planning permission. Despite this, mansard conversions are common in London — they are a well-established type and many councils in London (particularly Kensington, Chelsea, Wandsworth, Islington, Camden) are familiar with them and routinely grant permission for rear mansards on terraced houses that comply with the standard mansard form.

*Typical cost*: £20,000–£45,000 for the mansard roof structure, in addition to the rest of the loft conversion cost.

**Type 5 — Conservation roof light / traditional dormer**:

• In conservation areas and on listed buildings, the standard box dormer is often not acceptable. Many conservation area supplementary planning documents specify that dormers in conservation areas should be:
• •Traditional pitched-roof dormers with a steeply pitched gable or hip roof
• •Smaller and set back from the eaves and the gable ends
• •Clad in natural materials (lead, slate, zinc) rather than upvc or aluminium
• •Limited to one or two dormers rather than a full-width box

The 'conservation dormer' or 'traditional dormer' creates less space than a box dormer (it is smaller and has a pitched roof above the window head), but it is the planning-appropriate type in conservation areas.

*Typical cost*: £4,000–£10,000 per traditional dormer (smaller structure, more complex pitched roof geometry, high-quality cladding materials).

**Permitted development rules for dormers (England 2024)**:

• Under the General Permitted Development Order (GPDO) as amended, additions to a dwelling house roof are permitted development (Class B) provided:
• •The materials used are similar in appearance to the existing house
• •The dormer does not exceed the highest part of the original roof
• •The dormer does not overhang the outer face of the wall of the original house
• •On a property with a designated highway-facing elevation, no addition is permitted to the front slope (the elevation facing the highway)
• •The cubic content of the resulting roof space does not exceed 40m³ (for a terraced house) or 50m³ (for any other house)
• •The dormer does not include balconies, verandas, or raised platforms

• Important PD limitations specific to London:
• •*Conservation area properties*: All loft alterations (including rear dormers) require planning permission in conservation areas in London — PD rights for roof additions are removed by the conservation area designation
• •*Article 4 Directions*: Some London boroughs have Article 4 Directions that remove PD rights for roof additions even in non-conservation-area streets — check with the local planning authority
• •*Listed buildings*: PD rights do not apply to listed buildings — listed building consent is required for any external alteration
• •*Flats*: PD rights do not apply to flats — a flat in a converted terraced house cannot use PD to add dormers; planning permission is always required

**Local planning authority policies**:

• Beyond PD rights, each London borough has its own Local Plan policies governing the design, appearance, and scale of roof additions. These policies:
• •Specify materials (slate cladding preferred over zinc in some conservation areas; lead on roofs of traditional dormers)
• •Specify setbacks from the ridge and from the gable ends (typically 300–500mm setback from the ridge; 300mm setback from the gable ends)
• •Specify maximum dormer height relative to the existing ridge
• •Specify appearance (flat-roofed box dormers permitted in some areas, mansard required in others)

• Common London borough policies:
• •*Inner London boroughs (Kensington, Chelsea, Westminster, Camden, Islington, Lambeth, Wandsworth)*: Strong preference for mansard or traditional dormers on conservation area properties; flat-roof box dormers sometimes permitted in non-conservation streets
• •*Outer London boroughs (Enfield, Havering, Bromley, Kingston, Richmond)*: More permissive approach in non-conservation streets; rear box dormers widely accepted

**Pre-application advice for dormer planning**:

For any dormer that requires planning permission, a pre-application advice meeting with the local planning authority is strongly recommended — typically £0–£250 for a householder application. The pre-application advice will identify whether the proposed dormer type is likely to be acceptable without a full application, potentially saving the cost of a refused application (£258 application fee non-refundable) and the delay of appeal.

A local planning consultant (typically £500–£1,000 for a planning assessment) can also assess PD rights and permitted development certificate eligibility before work starts — useful where there is uncertainty about PD status (conservation area proximity, removed PD rights via Article 4).

**Dormer costs in context**:

The dormer structure is one element within the full loft conversion cost. The table below shows typical all-in costs for different loft conversion types in London (2025), including the dormer, structural work, staircase, insulation, electrics, plumbing (if bathroom included), plastering, and decoration:

| Loft conversion type | Typical all-in cost (London 2025, inc. 1 bedroom and bathroom) | |---|---| | Velux / roof light only (no structural dormers) | £18,000–£30,000 | | Single rear box dormer (small, 1 window) | £30,000–£45,000 | | Full-width rear box dormer | £38,000–£65,000 | | Hip-to-gable (semi-detached) + rear dormer | £55,000–£85,000 | | L-shaped dormer (hip-to-gable + large rear dormer) | £55,000–£90,000 | | Mansard dormer (full rear slope conversion) | £65,000–£110,000 | | Two dormers (front and rear — e.g. a master suite with en-suite and dressing room) | £55,000–£95,000 |

Note: These are all-in costs including structural work, waterproofing, insulation to Part L, staircase, electrics, plumbing/en-suite bathroom (mid-spec), plastering, decoration, and second-fix carpentry. They do not include second-fix materials (tiles, sanitaryware) which under RCB's standard model are client-supplied.

**Dormer roofing materials**:

• *Flat roof over box or shed dormer*:
• •*EPDM (rubber) membrane*: Low cost, 25+ year life, fully bonded installation; typically the most cost-effective flat roof for dormers — see `flat-roof-extension-guide`
• •*GRP (fibreglass)*: Seamless moulded finish, long-lasting, slightly higher upfront cost than EPDM; popular for dormers where a very clean finish is desired
• •*Single-ply TPO/PVC*: Mechanically fixed membrane; good for larger flat roof areas

• *Dormer cheeks (side and front face cladding)*:
• •*Natural slate*: Premium appearance, long life, matches existing Victorian slate roof; preferred in conservation areas
• •*Zinc (standing seam)*: Modern appearance, 40+ year life, low maintenance; widely used for contemporary dormers and box dormers in inner London
• •*Fibre cement slate*: More affordable than natural slate, very similar appearance; widely used in standard loft conversions
• •*Tile hanging*: Using small clay or concrete tiles hung on the dormer cheeks — popular in some south London and outer London areas where brick and tile are the vernacular material
• •*Timber cladding (larch, cedar, oak)*: Used for modern or contemporary architectural schemes; requires regular maintenance or pre-treatment for long life

**Structural considerations**:

• All dormers involve structural work to the existing roof — the existing rafters must be modified (cut back or trimmed) to create the dormer opening, new trimmer rafters must be installed either side of the opening, and the dormer wall plates and roof structure must be designed. A structural engineer must calculate:
• •The loads imposed by the dormer on the existing roof structure
• •The size and specification of the trimmer rafters and new structural members
• •The ridge beam or other structural support if the dormer spans a significant proportion of the roof width
• •Any changes required to the existing wall plate, ceiling joists, or party wall structure

Structural engineer fee for a typical London loft conversion: £700–£2,000.