Loft Conversion Types Explained: Dormer, Mansard, Hip-to-Gable & Velux

Why the type of loft conversion matters

A loft conversion is one of the most effective ways to add space and value to a UK home - but the type of conversion you choose affects everything: how much useable floor area you gain, what the space feels like to live in, whether you need planning permission, and how much it costs. Choosing the wrong type for your property wastes money and delivers a compromised result.

The five main types of loft conversion in the UK are Velux (rooflight), rear dormer, hip-to-gable, mansard, and L-shaped dormer. Each has distinct structural requirements, planning implications and cost profiles. Your property type, roof shape, budget and the space you need should drive the decision - not the other way around.

At RCB Design & Build, we assess every loft project on its structural merits before recommending a conversion type. As FMB and TrustMark accredited contractors with extensive experience across London and the South-East, we have delivered every type listed below and understand the real-world trade-offs of each.

Velux (rooflight) loft conversions

A Velux or rooflight conversion is the simplest and most affordable type. It involves adding roof windows to the existing roof slope without altering the roof structure itself. The floor is strengthened, a staircase is installed, and the space is insulated, boarded and finished - but the roofline stays exactly as it is.

Cost in 2026: GBP 45,000-GBP 60,000 for a bedroom and en-suite in London and the South-East. This is the entry-level price point for a genuine loft conversion (not a loft boarding job with a ladder).

Pros: lowest cost, fastest build (typically 6-8 weeks on site), almost always falls within permitted development, minimal disruption to neighbours, and preserves the existing roof character - important in conservation areas where dormer alterations may be refused.

Cons: only works where existing ridge height provides adequate standing headroom (typically 2.2 metres minimum at the ridge). Useable floor area is limited by the roof slope - you lose significant space at the eaves. Not suitable for properties with low-pitch roofs or modern trussed-rafter construction.

Best suited to: Victorian and Edwardian terraces with steep-pitch traditional cut roofs and adequate ridge height. Also works well on 1930s semi-detached houses with generous loft voids.

Rear dormer loft conversions

A rear dormer extends outward from the rear roof slope, creating a flat-roofed or slightly pitched box that dramatically increases internal headroom and useable floor area. The rear dormer is by far the most common loft conversion type on London terraces because it maximises space within permitted development limits.

Cost in 2026: GBP 55,000-GBP 85,000 for a standard single-bedroom-plus-en-suite rear dormer. A full-width rear dormer accommodating a second bedroom or study area pushes towards GBP 90,000-GBP 100,000.

Pros: significant increase in useable floor area compared to a Velux conversion, full standing headroom across most of the new space, usually possible under permitted development for houses (subject to volume limits), and versatile enough to accommodate a range of layouts from a single en-suite bedroom to a two-room arrangement.

Cons: changes the appearance of the rear of the property (though this is rarely visible from the street), requires more structural work than a Velux conversion, and the flat-roof element needs high-quality detailing to prevent long-term waterproofing issues. Cost is significantly higher than Velux.

Best suited to: mid-terrace and end-of-terrace Victorian and Edwardian houses across London. Also effective on 1930s semi-detached properties where a rear dormer can be combined with internal reconfiguration.

Hip-to-gable loft conversions

A hip-to-gable conversion is specifically for properties where the side roof slopes inward (a hipped roof) rather than meeting a vertical gable wall. The hipped end is rebuilt as a vertical gable, creating a full-height wall where there was previously a sloping roof. This substantially increases the useable loft floor area.

Cost in 2026: GBP 65,000-GBP 95,000 for a hip-to-gable conversion on its own. When combined with a rear dormer - the most common arrangement - costs typically run GBP 75,000-GBP 110,000. The combination of hip-to-gable plus rear dormer delivers the maximum practical floor area for semi-detached and end-of-terrace properties.

Pros: transforms otherwise unusable hipped roof space into full-height, full-width rooms. Combined with a rear dormer, the floor area rivals a mansard conversion at lower cost. Often possible under permitted development for houses, though the gable end may require planning consent in some boroughs.

Cons: only applicable to hipped-roof properties (not relevant for mid-terrace houses with party walls on both sides). Requires substantial structural work to rebuild the roof end. The new gable wall is visible from the street, so design and material matching are important.

Best suited to: 1930s semi-detached houses with hipped roofs - the classic candidate for this conversion type. Also works well on detached houses and end-of-terrace properties with a hipped side roof.

Mansard loft conversions

A mansard conversion is the most extensive and most expensive type. It involves rebuilding the rear roof slope (and sometimes both slopes) at a near-vertical angle of 72 degrees, with a flat roof section at the top. This creates the maximum possible internal volume and the best ceiling heights of any loft conversion type.

Cost in 2026: GBP 85,000-GBP 120,000+ in London and the South-East. Complex mansards on larger properties or those requiring full planning permission can exceed GBP 140,000. The premium over a standard dormer is driven by the scale of structural work - effectively the entire rear roof is demolished and rebuilt.

Pros: delivers the maximum useable floor area and the best ceiling heights. Creates rooms that genuinely feel like part of the house rather than a converted attic. Allows for larger, more flexible layouts - commonly two bedrooms, a bathroom and a landing area. Adds the most value to the property.

Cons: almost always requires full planning permission (rarely falls within permitted development). Significantly more expensive than other types. Longer build programme - typically 12-16 weeks on site. More disruptive during construction due to the scale of roof demolition and rebuild. May face planning resistance in conservation areas.

Best suited to: properties where maximum space is the priority and budget allows. Common on Victorian and Edwardian terraces in London where property values justify the investment. Also used on period properties where a sympathetic mansard design can gain planning approval that a flat-roofed dormer might not.

L-shaped dormer loft conversions

An L-shaped dormer combines a rear dormer with a side dormer that wraps around the rear outrigger - the small rear projection found on many Victorian and Edwardian terraces. The result is an L-shaped floor plan that captures space above both the main rear roof and the outrigger roof, delivering significantly more floor area than a standard rear dormer alone.

Cost in 2026: GBP 75,000-GBP 105,000 depending on the property size and specification. The additional cost over a standard rear dormer is modest relative to the extra space gained, making this excellent value where the property layout allows it.

Pros: captures the dead space above the rear outrigger that a standard rear dormer misses. Often delivers enough floor area for two bedrooms plus a bathroom without needing a full mansard. Makes efficient use of the existing property footprint.

Cons: only possible on properties with a rear outrigger at the correct level. Structural complexity is higher than a standard rear dormer due to the valley junction between the two dormer sections. Waterproofing at the L-junction requires experienced detailing.

Best suited to: Victorian terraces with rear outriggers - extremely common across London, particularly in E, SE, N and NW postcodes. If your property has a two-storey rear addition with its own lower roof, an L-shaped dormer is likely the best value conversion type available to you.

How to decide which type is right for your property

The decision starts with your property, not your preferences. A mid-terrace Victorian house with good ridge height and a rear outrigger is a natural candidate for an L-shaped dormer. A 1930s semi with a hipped roof points toward hip-to-gable plus rear dormer. A property in a conservation area with planning constraints may only be able to achieve a Velux conversion.

Budget is the second filter. If your total budget including all fees and contingency is under GBP 65,000, a Velux conversion is the realistic option. Between GBP 65,000 and GBP 90,000, a rear dormer or hip-to-gable becomes achievable. Above GBP 90,000, the full range of options opens up including mansard.

The third factor is what you need the space for. A single extra bedroom with en-suite can be achieved with any type. Two bedrooms plus a bathroom typically requires a dormer, hip-to-gable or mansard. A home office, playroom or media room benefits from the generous proportions of a mansard.

We recommend starting with a free project review where we visit the property, assess the existing roof structure, check planning constraints, and recommend the conversion type that delivers the best outcome for your budget and brief. There is no obligation and no charge for this initial assessment.