Key takeaways
- The four main types of loft conversion are dormer, hip-to-gable, mansard and rooflight (Velux).
- A loft conversion can add up to 24% to a property's value when it creates a double bedroom with a bathroom (Which?, citing Nationwide, 2025).
- Rooflight conversions are the cheapest because they do not alter the roof structure, while mansard conversions are the most involved.
- Permitted development allows up to 40 cubic metres of extra roof space for terraced houses and 50 cubic metres for detached and semi-detached homes (Planning Portal).
- Most loft conversions take roughly 6 to 12 weeks on site, depending on the type and complexity.
There are four main types of loft conversion used in UK homes: dormer, hip-to-gable, mansard and rooflight (also called Velux). The right one for your house depends on your roof shape, your budget and how much head height you need. This guide walks through each type, what it suits and what to expect during the build, so you can match the conversion to your home before you call a builder.
Loft conversions remain one of the most cost-effective ways to add a bedroom, office or bathroom without losing garden space. A well-planned conversion can add up to 24% to a property's value when it creates a double bedroom with an en-suite in a three-bedroom house, according to Which? citing Nationwide research from 2025. That kind of return is why so many homeowners in Northampton and across the UK start here rather than moving.
What are the main types of loft conversion?
The four standard types are dormer, hip-to-gable, mansard and rooflight. Each changes the roof in a different way, from adding nothing structural (rooflight) to rebuilding an entire roof slope (mansard). Cost, head height and planning rules all shift with the type you choose.
Think of them as a ladder. Rooflight is the simplest and cheapest. Dormer adds a box of extra space. Hip-to-gable rebuilds a sloping side into a vertical wall. Mansard reshapes the whole roof for maximum room. The best fit is the one that gives you usable space for the least disruption to your roof.
How does a dormer conversion maximise headroom?
A dormer conversion adds a structural box that projects vertically out from the sloping roof, creating flat walls and full standing height inside. It is the most popular type in the UK because it suits a wide range of houses and delivers a big jump in usable floor space.
The flat roof and vertical window of a dormer turn an awkward triangular loft into a proper room. You gain headroom exactly where you need it: at the centre and edges of the space where a sloped ceiling would otherwise force you to stoop. A rear dormer on a terraced or semi-detached house is often the sweet spot between cost and the amount of room you get back.
Dormers usually fall under permitted development, but the rules are specific. The dormer must be set back at least 20 cm from the original eaves, and any side-facing window that opens has to sit 1.7 metres above the floor (Planning Portal). A reputable builder will design to these limits so you avoid a planning application where possible.
Is a hip-to-gable conversion right for a semi-detached or detached home?
A hip-to-gable conversion extends the sloping "hipped" side of a roof outwards to form a vertical gable wall, filling in the wasted triangular space at the end of the roof. It works only on houses that have a hipped roof, which rules out most terraces but suits many semi-detached and detached properties.
By squaring off that sloped end, you turn a cramped corner into full-height floor area. Hip-to-gable is frequently combined with a rear dormer to create a generous double bedroom with an en-suite. The two together can transform a modest loft into the largest room in the house.
Because this type changes the roof's outline and raises a new gable wall, it involves more structural work than a simple dormer. On many detached and semi-detached homes it still falls within permitted development, which allows up to 50 cubic metres of additional roof space (Planning Portal). End-of-terrace houses with a hip can sometimes qualify too.
Why do mansard conversions suit terraced properties?
A mansard conversion replaces one slope of the roof with a steep, near-vertical wall and a flat roof above. It creates almost a full extra storey, which is why it is a favourite for terraced houses that need maximum space from a tight footprint.
Mansards deliver the most internal room of any loft type. The steep rear wall gives near-vertical headroom across most of the floor, so you rarely waste space to sloped ceilings. For a narrow Victorian or Edwardian terrace, that extra volume can mean the difference between a cramped box room and a proper master suite.
The trade-off is cost and permissions. A mansard rebuilds a large part of the roof, so it is the most involved and expensive type, and it almost always needs full planning permission rather than relying on permitted development. For terraced homes, that extra investment often pays off in both living space and resale value.
How do rooflight (Velux) conversions keep costs down?
A rooflight conversion, often called a Velux conversion after the common window brand, keeps the existing roof shape and simply adds windows set flush into the slope, along with a new floor, stairs and insulation. Because nothing structural changes on the roofline, it is the most budget-friendly option.
This type works best when your loft already has enough head height to stand up in comfortably. There is no extension outwards or upwards, so you keep the roof's original profile and avoid the cost of rebuilding it. That also makes rooflight conversions the least likely to need planning permission, since they stay within the existing volume.
The limitation is space. You only get the room the roof already offers, so a shallow or low loft may not convert well without a dormer added later. If your roof has the height, though, a rooflight conversion is the fastest and cheapest route to an extra room.
How do you choose the right loft type for your roof shape?
Start with your roof shape, because it decides which types are even possible. A hipped roof opens the door to hip-to-gable. A terrace usually points towards a mansard or a rear dormer. A roof with generous existing height may need only a rooflight conversion. Match the type to the roof first, then weigh cost and the space you want.
Ask three questions in order:
- Does my loft already have standing height? If yes, a rooflight conversion may be enough.
- Do I need more headroom or floor space? A dormer adds both, and hip-to-gable adds floor area on a hipped roof.
- Do I want a full extra storey? On a terrace, a mansard delivers the most room.
Budget then narrows the choice. Rooflight sits at the bottom, dormer and hip-to-gable in the middle, and mansard at the top. Permitted development allows up to 40 cubic metres of extra space for terraced houses and 50 cubic metres for detached and semi-detached homes, which can shape what you build before any planning application is needed (Planning Portal). A local contractor who knows your area's housing stock can confirm what your roof allows. At AAS Group Construction in Northampton, this roof assessment is the first step before any design work begins.
If you are weighing a loft against other ways to add space, our guide to a house extension in Northampton covers costs, ideas and the process side by side.
What timescales should you expect during the build?
Most loft conversions take roughly 6 to 12 weeks on site, with the type and complexity setting where you land in that range. A straightforward rooflight conversion is the quickest, while a mansard or a combined hip-to-gable and dormer build takes the longest because of the structural work involved.
A typical build runs in clear stages. First come scaffolding and structural work, including new floor joists and any roof alterations. Next the shell is made weathertight, then stairs, insulation, plasterboard and first-fix electrics and plumbing go in. The final weeks cover plastering, decorating, flooring and fitting out any bathroom. Building control signs off the work at key points for safety and fire rules.
Expect some disruption, mostly during the noisy structural phase early on. A good contractor sequences the work to keep your home liveable and gives you a week-by-week plan up front. Clear timescales and a single point of contact make the difference between a stressful project and a smooth one.
Where to go from here
The quickest way to know which loft conversion fits your home is to have someone look at your actual roof, not a generic cost table. Book a site assessment with a local contractor who can measure your head height, check your roof shape and tell you straight which types are possible and what each would involve. If you are in or around Northampton, AAS Group Construction can carry out that assessment and walk you through your options before you commit to a design.
Frequently asked questions
Which type of loft conversion is cheapest?
A rooflight or Velux conversion is usually the cheapest because it keeps the existing roof shape and only adds windows, floor and insulation. There is no structural extension to the roofline, so labour and materials cost less.
Do I need planning permission for a loft conversion?
Many loft conversions fall under permitted development, which allows up to 40 cubic metres of extra space for terraced houses and 50 cubic metres for detached and semi-detached homes. Mansard conversions and homes in conservation areas usually need full planning permission.
How much value does a loft conversion add?
A loft conversion can add up to 24% to a property's value when it adds a double bedroom and a bathroom to a three-bedroom house, according to Which? citing Nationwide research from 2025. The exact uplift depends on location and finish.
How long does a loft conversion take to build?
Most loft conversions take roughly 6 to 12 weeks on site. A simple rooflight conversion sits at the lower end, while a mansard or hip-to-gable build with structural work takes longer.
Which loft conversion suits a terraced house?
A mansard conversion suits many terraced houses because it replaces a slope of the roof with a near-vertical wall, creating a full extra storey. A rear dormer is a common lower-cost alternative for terraces.
