Dormer Extensions London 2026: Types, Costs & Planning Rules

What Is a Dormer Extension?

A dormer is a structural addition to an existing roof that creates vertical walls and a new roof section, projecting outward from the main roof plane. It converts previously unusable sloped loft space into full-height, habitable rooms. In London, dormers are almost universally added to the rear slope of the roof — front dormers are restricted under permitted development rules and rarely approved on conservation area properties.

London's housing stock is particularly well-suited to dormer loft conversions. Victorian and Edwardian terraces — which make up a substantial proportion of homes in inner south, east, and north London — typically have steeply pitched roofs with enough ridge height to create a full storey of habitable space once a dormer is added. A standard loft conversion with a rear dormer on a two-storey Victorian terrace will typically add one or two bedrooms and a bathroom, meaningfully increasing the usable floor area of the home without touching the footprint of the building.

The key planning framework for dormers is Class B of the General Permitted Development Order (GPDO), which covers additions to roofs that enlarge a dwelling. Understanding Class B — its volume limits, design conditions, and exclusions — determines whether your dormer can proceed without a planning application.

Types of Dormer Extension

There are five main dormer types used on London properties. Each has different characteristics in terms of the space created, cost, planning implications, and suitability for different roof forms and house types.

Flat roof dormer

The flat roof dormer is by far the most common type in London. It projects from the rear of the main roof and has a flat (or very slightly sloped) roof section. The vertical cheeks — the side walls of the dormer — are typically clad in zinc, lead, or standing seam metal. The flat roof sits flush with or just below the existing ridge height, and the face of the dormer is set back at least 20cm from the original eaves line as required by permitted development rules.

A full-width flat roof dormer on a Victorian terrace creates the maximum possible floor area and headroom across the entire loft. It is the most cost-effective option per square metre of usable space created. Most London loft conversions with a rear dormer use this form. The dormer face is typically glazed with one or two windows, or incorporates full-width glazed doors onto a Juliet balcony.

Shed dormer

A shed dormer is similar to a flat roof dormer in form, but its roof slopes down at a shallow angle — typically 10–15 degrees — rather than being truly flat. It suits properties with a shallower main roof pitch where a flat roof might look disproportionate, or where a modest slope improves water run-off. In terms of space created and cost, it is very close to a flat roof dormer. The distinction is primarily one of aesthetics and roof geometry.

Hip-to-gable dormer

A hip-to-gable conversion extends the roof of a semi-detached or end-of-terrace house by converting the inward-sloping hip end of the roof into a vertical gable wall. This dramatically increases the usable floor area of the loft, particularly on the side of the property that would otherwise taper to almost nothing under the hip. Hip-to-gable conversions are commonly combined with a rear flat roof dormer to create a full double-sided loft conversion.

Hip-to-gable conversions are not available to mid-terrace properties, which have gable walls on both sides already. They are popular on 1930s and post-war semis in outer London boroughs such as Ealing, Barnet, Sutton, and Kingston.

L-shaped dormer

The L-shaped dormer is specifically designed for Victorian and Edwardian terraced properties with a rear outrigger — a lower two-storey back addition that runs behind the main part of the house. These outriggers are a defining feature of the long back addition typical of inner London terraces. The L-shaped dormer extends across the rear of the main roof and wraps around and over the outrigger roof, creating a continuous dormer that runs in an L-shape when viewed from above.

The result is significantly more floor area than a simple rear dormer alone — typically 40–60m² of usable loft space rather than 20–30m². L-shaped dormers are the most popular option for inner London Victorian terraces in boroughs such as Hackney, Lambeth, Lewisham, and Southwark. They typically accommodate two bedrooms and a bathroom within the converted loft, creating a genuinely self-contained upper storey.

Gable-end dormer

A gable-end dormer is a smaller, traditional dormer window form that projects from the roof but does not run the full width of the building. These are more characteristic of rural and period architecture, and are less common in London loft conversions where maximising floor area is the primary goal. They are sometimes used in conservation areas where a full-width flat roof dormer would not be approved, as their traditional form is more sympathetic to Victorian and Edwardian street characters.

Permitted Development Rules for Dormers

Dormers fall under Class B of the General Permitted Development Order, which permits additions and alterations to roofs that enlarge a dwelling. The key rules are as follows.

Volume limits

The volume of the dormer — measured as the addition to the existing roof space — must not exceed:

• 40 cubic metres for terraced houses (including end-of-terrace)
• 50 cubic metres for detached and semi-detached houses

These are lifetime allowances — any previous loft works count towards the limit. A standard full-width rear dormer on a mid-terrace Victorian house typically uses 20–30m³, comfortably within the 40m³ allowance. An L-shaped dormer, however, often exceeds 40m³ due to the additional volume over the outrigger, which is why many L-shaped conversions require a planning application.

Design conditions

• The dormer must not exceed the height of the existing ridge line
• The dormer must be set back at least 20cm from the original eaves line on all sides
• Materials must be similar in appearance to those on the existing house
• Side-facing windows must be obscure-glazed and non-opening below 1.7m from floor level (to protect neighbour privacy)
• No verandas, balconies, or raised platforms are permitted under Class B (a Juliet balcony with inward-opening doors is acceptable)

When permitted development does not apply

Class B permitted development rights are removed or restricted in the following situations:

• Conservation areas: Roof alterations visible from a highway require planning permission in designated conservation areas
• Listed buildings: All alterations require listed building consent and full planning permission
• Article 4 directions: Specific Article 4 directions in some boroughs remove Class B rights, requiring planning permission for all roof works
• Flats and maisonettes: Class B does not apply to conversions or flats — only to houses
• Volume already used: If previous loft works have already used the full 40m³ or 50m³ allowance, a new dormer requires planning permission

Dormers That Need Planning Permission

Where a dormer falls outside Class B — because it exceeds volume limits, is in a conservation area, or involves a listed building — a householder planning application is required. The application fee is £258 (the standard householder fee for England, set in December 2023). Most London councils determine householder applications within 8 weeks.

For dormers in conservation areas, the key design considerations are:

• Rear dormers are far more likely to be approved than front dormers. Planning officers prioritise preserving the uniform roofline visible from the street.
• Materials matter. Metal-clad cheeks in zinc or lead, with slate or matching tiles retained at the sides, are typically acceptable. Render or timber cladding may raise objections in strict conservation areas.
• Set-back from the ridge and eaves. Maintaining a clear margin of original roof slope at the top and bottom of the dormer improves the relationship with the existing roofline and is viewed favourably by heritage officers.
• Pre-application advice is strongly recommended in conservation areas in boroughs such as Camden, Islington, Hackney, and Kensington & Chelsea. These can cost £100–£250 but significantly reduce the risk of a refusal.

Dormer Extension Costs in London 2026

All costs below are inclusive of VAT at 20% and reflect 2026 market rates for Greater London contractors. Costs in inner London boroughs run 10–15% higher than outer London; central boroughs such as Camden, Kensington, and Westminster may be 20–25% higher than the figures shown.

What drives cost variation

• Number of rooms: Adding a bathroom to the converted loft adds £12,000–£22,000 for plumbing, waterproofing, and tiling. A simple bedroom only is at the lower end of the cost ranges above.
• Staircase: A new loft staircase costs £4,000–£9,000 depending on specification and whether it requires reconfiguring an existing landing or bedroom.
• Glazing specification: Standard double-glazed dormer windows are included in the base cost. Juliet balconies, roof terraces, or high-specification glazing such as triple glazing or automated roof windows add £3,000–£12,000.
• Structural complexity: Where existing floor joists need strengthening or replacing, steel beams are required, or the party walls need extensive repair, structural costs increase significantly.
• Site access: In dense terraced streets, skips and deliveries require road licences. Scaffolding in narrow south London alleyways or on busy inner-city streets adds £2,000–£5,000 over suburban equivalents.

Structural Considerations for Dormer Extensions

Every dormer conversion requires structural input. A structural engineer is not optional — it is a requirement under building regulations. The structural work associated with a dormer conversion falls into three areas: floor strengthening, roof structure alteration, and load transfer.

Floor strengthening

Victorian loft floors were built as ceiling joists — designed to hold only the ceiling plaster below, not to carry live loads from occupants and furniture. Before the loft can be used as a habitable room, the floor structure must be upgraded. This typically involves either doubling the existing joists (sistering new joists alongside old ones) or replacing them entirely with deeper, stronger sections. In some cases, steel beams are introduced to span between party walls and carry the new floor. The engineer specifies the solution based on the span, the condition of the existing timbers, and the planned use of the space.

Roof structure alteration

Creating the dormer opening requires removing sections of the existing roof structure — rafters, ridge boards, and purlins. This must be done with temporary propping in place. New structural elements (ridge beams, trimmer rafters, and dormer headers) carry the loads that the removed elements previously bore. On Victorian terraces, the roof structure is typically cut-and-pitched traditional rafter construction, which is generally straightforward to alter. Older properties with complex or previously modified roof structures may require more involved assessment.

Load transfer to party walls

In terraced properties, the structural loads from the new dormer are often transferred to or through the party walls shared with neighbours. New steel beams bearing onto party walls, new padstones, and changes to how loads are distributed through the party wall structure are common features of dormer loft conversions on Victorian terraces. This is directly relevant to the party wall process described below.

Party Wall Considerations

The Party Wall etc. Act 1996 applies to most dormer loft conversions on terraced and semi-detached properties in London. The Act requires that you serve written notice on any adjoining owner before carrying out certain works — specifically, any works that directly affect the party wall structure.

For dormer conversions, the party wall trigger typically arises when:

• Steel beams or padstones are to be built into or bearing onto the party wall
• The loft floor is to be carried by or bears against the party wall
• The dormer cheeks are to be built on or against the party wall at roof level

A mid-terrace property has two party walls — one on each side. Both neighbours must be served notice. Notices must be served at least 2 months before starting structural party wall works. If a neighbour consents in writing, no party wall award is needed. If they dissent or do not respond within 14 days, the Act requires the appointment of a surveyor (or a jointly appointed agreed surveyor) to draw up a party wall award.

In practice, on a London Victorian terrace, you should budget for party wall surveyors for both neighbouring owners. A surveyor costs £1,200–£2,500 per neighbouring owner — if both neighbours appoint their own surveyors, this can add £3,000–£5,000 to project costs. Start the party wall process early: allow at least 3–4 months from serving notice to having a party wall award in place before construction begins.

Dormer Conversion Timeline

A typical dormer loft conversion on a London Victorian terrace, proceeding under permitted development, takes around 6–9 months from first consultation to completion. The table below shows how this breaks down.

Getting the Most From Your Dormer

• Maximise headroom by designing the staircase carefully. The staircase is often the hardest element of a loft conversion to fit. A well-designed stair that takes space from an existing landing or bedroom — rather than a cramped new compartment — allows the loft room to use its full floor area and improves the feel of the whole floor.
• Consider a Juliet balcony on the dormer face. A Juliet balcony — inward-opening glass doors with a fixed balustrade — brings in significantly more light than a standard dormer window and creates a sense of connection to the garden below. Under Class B permitted development, full cantilevered balconies are not allowed, but Juliet balconies are. They add approximately £3,000–£6,000 to the cost.
• Add a bathroom if it is at all feasible. A loft conversion with an en-suite bathroom adds more value per pound spent than almost any other home improvement in London. The incremental cost of adding a bathroom to the loft conversion programme — rather than as a separate future project — is significantly lower than doing it later.
• Get a Lawful Development Certificate after completion. If your dormer was built under permitted development, a Lawful Development Certificate (LDC) confirms this in writing. It costs £206 (half the householder fee) and makes the permitted development status clear in any future property sale. Without one, buyers' solicitors often raise queries that can hold up a sale.
• Insulate to current standards, not minimum. Loft rooms lose heat rapidly through roof slopes if underinsulated. Using 150mm of rigid insulation between rafters plus 50mm below meets Part L and reduces heating bills significantly. The marginal cost difference over minimum specification insulation is small relative to the whole project budget.
• On a Victorian terrace, check whether an L-shaped conversion is viable. If your property has a rear outrigger — visible as the lower back addition typical of inner London terraces — an L-shaped dormer may add 30–40% more floor area for only 15–20% more cost compared to a simple rear dormer. It is worth pricing both options before committing to a design.