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Considering a loft conversion? This guide covers every type — from dormer and mansard to hip-to-gable and Velux — with costs, pros, cons, and planning considerations.
A loft conversion is one of the most cost-effective ways to add living space to a London home. Unlike extensions, which eat into valuable garden space, a loft conversion makes use of the empty volume above your head. In a city where the average price per square foot is among the highest in the world, adding a bedroom and en-suite in the loft can add £100,000 or more to the value of a typical London property. There are several types of loft conversion, and the right one for your home depends on the existing roof structure, the available head height, planning constraints, and your budget.
A Velux or rooflight conversion is the simplest and most affordable type. It involves adding roof windows (Velux is the best-known brand) to the existing roof slope without altering the external shape of the roof. This type works well when there is already sufficient head height in the loft — generally at least 2.2 metres from the top of the floor joists to the ridge. Because the roof shape is unchanged, Velux conversions almost always fall within permitted development rights, making them the fastest route to extra space. Costs typically range from £25,000 to £40,000 in London.
The dormer conversion is the most popular type in London. A dormer is a box-shaped structure that projects out from the rear roof slope, creating additional headroom and usable floor space. A full-width flat-roof dormer across the back of the house is the most common configuration, as it maximises the gain in floor area. Dormers can usually be built under permitted development on the rear of a property, though there are restrictions on size, height, and materials. Front dormers and dormers on side elevations typically require planning permission. Expect to pay £35,000 to £55,000 for a standard rear dormer in London.
Good to Know
A rear dormer loft conversion typically adds 20–30 square metres of floor space to a standard London terraced house. This is usually enough for a double bedroom, an en-suite bathroom, and built-in storage.
Hip-to-gable conversions are specifically designed for properties with hipped roofs — where the roof slopes inward on the side as well as the front and back. This type is very common on 1930s semi-detached houses. The conversion involves extending the sloping hip at the side of the house outward to create a vertical gable wall, which dramatically increases the usable space in the loft. A hip-to-gable conversion is often combined with a rear dormer to maximise the total floor area. This type usually falls under PD for semi-detached houses. Costs range from £40,000 to £60,000.
A mansard conversion involves changing the entire roof structure, typically raising the party wall and building a new roof with a near-vertical slope (at least 72 degrees) and a flat top. This creates maximum internal space and the most usable floor area of any conversion type. Mansards are particularly popular on Victorian and Edwardian terraces in inner London boroughs. However, because they fundamentally alter the shape of the roof, mansard conversions almost always require full planning permission. They are also the most expensive type, typically costing £50,000 to £75,000 or more in London.
An L-shaped conversion is used on Victorian terraced houses with a rear outrigger — the two-storey projection at the back that gives these houses their distinctive L-shaped footprint when viewed from above. This type involves building dormers on both the main roof and the outrigger roof, creating a large L-shaped space that can accommodate two bedrooms and a bathroom. L-shaped conversions are complex but very effective at maximising space in a typical London terrace. Costs are typically in the range of £50,000 to £70,000.
Top Tip
Before committing to a loft conversion type, have a structural survey carried out. A surveyor can assess the existing roof structure, head height, and floor joists to confirm which types of conversion are feasible for your property.
All loft conversions must comply with Building Regulations, which include strict requirements for fire safety. When you create a habitable room on the second floor (third storey counting the ground floor), the staircase and landings below must form a protected escape route. This typically means fitting fire doors to all rooms opening onto the staircase and landing, ensuring walls and ceilings provide a minimum 30-minute fire resistance, and installing interlinked smoke alarms on every floor. In some cases, an alternative escape route such as an escape window may also be required.
A Velux (rooflight) conversion is the most affordable, typically costing £25,000–£40,000 in London. It works by adding roof windows to the existing roof slope without altering the external structure, which keeps construction time and costs to a minimum.
Many loft conversions, particularly rear dormers and Velux conversions, can be carried out under permitted development rights without planning permission. However, mansard conversions, front dormers, and work in conservation areas will usually require a planning application. Always check with your local council before starting.
A typical loft conversion takes 8–12 weeks on site. A straightforward Velux conversion may take 6–8 weeks, while a complex mansard or L-shaped conversion could take 12–16 weeks. Allow additional time before work starts for design, planning applications, and building regulations submissions.
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