Your existing roof shape narrows the options before budget enters the conversation. A hipped roof, a gable ended terrace and a low ridge each point towards a different conversion type, and choosing against that shape is how projects become expensive.
Most loft conversion London inquiries start with a preference and end with a survey. The survey usually wins.
Quick answer: A rear dormer suits gable ended terraces and is the most common London conversion, costing roughly £45,000 to £75,000. Hip to gable suits semi detached and detached houses with a hipped roof end. A mansard rebuilds the rear slope at around 70 degrees, delivers the most volume, costs £62,000 to £165,000 and always requires planning permission. A rooflight conversion is cheapest but needs around 2.2 metres of existing height above the ceiling joists.
What Roof Shape Do You Actually Have?
The three that matter are a gable ended roof, a hipped roof and a crown or shallow pitched roof.
A gable ended roof has vertical brick walls at each end and a slope front and rear. Most London terraces are built this way, and the rear slope is where a dormer goes.
A hipped roof slopes on all sides, which is common on interwar semi detached and detached houses across outer London. The sloping end is dead space, and converting it to a vertical gable is what unlocks the floor area.
A crown roof or shallow pitch has limited height at the ridge, which is where a mansard often becomes the only route to a usable room.
When Is a Rear Dormer the Right Loft Conversion?
A rear dormer suits a gable ended terrace with reasonable ridge height, which describes a large share of London’s Victorian and Edwardian stock.
It replaces part of the sloping rear plane with a vertical wall and a flat or shallow roof above, converting sloping dead space into full height space at the back of the plan, exactly where a bedroom and bathroom need to sit.
Costs typically run £45,000 to £75,000 in London, with a site duration of six to eight weeks and moderate structural work. An L shaped dormer, which wraps over a rear outrigger, runs £48,000 to £90,000 and takes eight to ten weeks.
The constraint is volume rather than design. A generous rear dormer commonly consumes 25 to 35 cubic metres, and the permitted development allowance is 40 cubic metres for a terraced house or 50 for a detached or semi detached one, including any previous roof additions.
When Does Hip to Gable Make Sense?
Hip to gable makes sense where the property has a hipped roof end, meaning a semi detached or detached house rather than a mid terrace.
The work rebuilds the sloping end as a vertical gable wall, which creates volume across the full width of the roof rather than just at the rear. It is frequently combined with a rear dormer, and that combination is the standard solution on a 1930s semi.
Expect £47,000 to £100,000 and seven to fourteen weeks, with heavy structural work since an entire roof plane is being reconstructed.
One useful technical quirk. Roof extensions other than hip to gable must be set back at least 20cm from the original eaves. Hip to gable is specifically exempt from that condition.
What Makes a Mansard Different?
A mansard rebuilds the rear roof slope as a near vertical wall, typically around 70 degrees, with a flat roof above. It produces substantially more usable floor area than any other type because almost the entire footprint gains full height.
It is also the most expensive and the most complex. London figures run £62,000 to £165,000 across eight to twelve weeks, with the heaviest structural work of the four.
A mansard loft conversion always requires planning permission. There is no permitted development route for it, which means the design will be assessed against local character whether or not the property sits in a conservation area.
That makes local precedent unusually important. Where neighbouring properties already carry approved mansards, the argument is considerably shorter.
Is a Rooflight Conversion Ever the Better Choice?
Yes, where the existing head height is genuinely generous and the budget is tight.
A rooflight conversion keeps the roof shape and inserts windows into the slope. At £18,000 to £48,000 across four to six weeks with minimal structural work, it is by a distance the cheapest route.
The threshold test is height. You generally need around 2.2 metres between ceiling joists and ridge to produce a compliant room, plus a layout that accepts a staircase without wrecking the floor below.
Where that height exists, paying for a dormer you do not need is not a virtue. Where it is marginal, the result is a room that technically exists and does not work.
How Do the Four Types Compare?
| Rooflight | Rear dormer | Hip to gable | Mansard | |
| London cost | £18k to £48k | £45k to £75k | £47k to £100k | £62k to £165k |
| Duration | 4 to 6 weeks | 6 to 8 weeks | 7 to 14 weeks | 8 to 12 weeks |
| Structural work | Minimal | Moderate | Heavy | Heaviest |
| Suits | Generous ridge height | Gable ended terrace | Hipped roof semi or detached | Low ridge, or maximum volume |
| Usable area gained | Ridge zone only | Substantial at rear | Full width | Greatest |
| Planning permission | Usually not needed | Usually permitted development | Usually permitted development | Always required |
| En suite feasible | Often not | Usually | Usually | Yes |
An en suite adds roughly £8,000 to £15,000 to any of them, and it is the item most often impossible in a rooflight conversion for want of head height.
Does Conservation Area Status Change the Answer?
Yes, decisively. Roof enlargement is removed from permitted development entirely on designated land, so dormers, hip to gable and mansards all require planning permission inside a conservation area.
Rooflights survive under a different class, but not on a roof slope fronting a highway. London has over 1,000 conservation areas across its boroughs, and several authorities layer Article 4 directions on top, so the address check comes before the roof survey.
Flats and maisonettes have no permitted development rights for extensions at all, regardless of designation.
What Conditions Apply Whichever Type You Choose?
Under permitted development, all of the following must hold simultaneously. No part above the highest point of the existing roof. No extension beyond the plane of the slope fronting a highway. Materials similar in appearance to the existing house. Side facing windows obscure glazed and non opening below 1.7 metres. No balconies, verandas or raised platforms, which take the whole conversion outside the class.
Building Regulations apply to every type in full. Structural calculations for new floor joists and any dormer structure, a protected stairway and escape route under Approved Document B, and headroom over the new staircase under Approved Document K.
A Lawful Development Certificate for proposed use costs around £274 against £548 for a full householder application, and it is the document a buyer’s solicitor will ask for.
Also Read: Kingston Homes Have Space Most London Properties Can Only Dream Of. Are You Using Yours?
