Loft Conversion Designs and Types: Which One Fits Your Roof
The four main loft conversion types in London are the Velux (rooflight) conversion, the dormer conversion, the hip-to-gable conversion, and the mansard conversion — each adding progressively more usable volume and cost, and each governed by different rules under GPDO 2015 Schedule 2 Part 1 Class B, Approved Document K stair headroom, and borough-level Article 4 restrictions. Which type your roof can actually take is decided by existing roof pitch and form, ridge height, headroom over the stair, and whether the property sits in a conservation area, not by preference alone.
Getting the type right at drawing stage — before a planning or Building Regulations submission — is what separates a scheme that gets approved first time from one that gets refused or has to be redesigned mid-process.
What are the main loft conversion types?
| Type | Added volume (typical) | Typical cost (drawings + build) | Planning route |
|---|---|---|---|
| Velux / rooflight only | None (no external volume increase — rooflights fitted flush to existing roof slope) | £25,000–£35,000 | Usually permitted development; no volume test applies |
| Dormer (rear or side) | Up to 40m³ (terrace) / 50m³ (semi/detached) under Class B, shared across all roof additions | £45,000–£65,000 | Often permitted development; full application if over volume limit or in a restricted area |
| Hip-to-gable | Counts toward the same 40m³/50m³ Class B allowance as dormers | £55,000–£75,000 (often combined with a rear dormer) | Often permitted development on semi-detached/detached hipped roofs; not available on mid-terrace |
| Mansard | Largest volume gain — typically exceeds Class B allowance | £65,000–£90,000+ | Usually requires full planning permission; near-flat 72° rear slope replaces the original roof form |
What does a Velux loft conversion actually involve?
A Velux, or rooflight, conversion fits windows flush into the existing roof slope without altering the roof's external volume or form. Because there's no extension to the roof structure, it's the type most likely to fall within permitted development everywhere, including in conservation areas, since Class B's volume and conservation-area restrictions are aimed at roof alterations that change the external envelope. It's also the cheapest and fastest route, but it only works where the existing roof pitch already gives close to 2.2m of head height at the ridge — without that, floor space usable to Approved Document K headroom standards (1.9m over the stair, 1.8m at the side) is too limited to be worthwhile.
How much volume does a dormer loft conversion actually add, and where can you build it?
A dormer conversion extends a section of roof outward, usually to the rear, creating near-vertical walls and full headroom across the new floor area — the most common loft type in London terraces and semis for exactly this reason. Under GPDO 2015 Schedule 2 Part 1 Class B, a terraced house can add up to 40m³ of additional roof volume as permitted development, and a semi-detached or detached house up to 50m³, provided materials are of a similar appearance to the existing house, there are no verandas, balconies or raised platforms, and side-facing windows are obscure-glazed with any opening 1.7m above floor level.
Class B does not apply to flats or maisonettes, and does not cover the principal (usually front-facing) elevation of a house in a conservation area. That matters directly in boroughs with extensive conservation area coverage — large parts of Wandsworth, Richmond upon Thames, and central Southwark — where a rear dormer may still be permitted development but the same design facing the street would need a full application.
What is a hip-to-gable conversion, and does every roof qualify?
A hip-to-gable conversion squares off a sloped (hipped) side roof into a vertical gable wall, which is only relevant — and only physically possible — on semi-detached and detached houses with a hipped roof; mid-terrace houses share party walls where there's no hip to convert. It's frequently combined with a rear dormer in the same scheme to maximise usable floor area, and the combined volume still counts toward the same 40m³/50m³ Class B allowance, so the two additions have to be designed and calculated together, not treated as separate volume budgets.
Croydon and Bromley, with substantial stock of 1930s semi-detached houses with hipped roofs, see a high proportion of hip-to-gable-plus-rear-dormer schemes for exactly this reason — the roof form suits it structurally and the combined volume typically stays within Class B limits.
Why does a mansard conversion usually need full planning permission?
A mansard conversion replaces most of the existing roof slope with a steep, near-vertical 72-degree face (traditionally to the rear, sometimes wrapping the full roof), maximising internal volume more than any other type — but because it fundamentally alters the roof form rather than adding a smaller dormer or gable feature within it, it typically exceeds the Class B volume allowance and changes the roof's external appearance enough that most London boroughs require a full householder planning application rather than permitted development. Councils must determine a standard householder application within 8 weeks of validation.
Mansards are common on London terraces where a matching mansard already exists next door or along the terrace — several conservation areas in Lambeth and Islington-adjacent streets have entire terraces finished this way, which can actually support a planning application on design-continuity grounds even where the individual scheme wouldn't otherwise be permitted development.
Where a mansard is proposed within a designated conservation area — for example a terrace within the Barnsbury Conservation Area in the London Borough of Islington, or streets within the Streatham Conservation Area in the London Borough of Lambeth — the council will typically also weigh the scheme against the relevant Local Plan and any conservation area SPD, in addition to the standard 8-week householder determination period once the application under the Town and Country Planning Act 1990 is validated.
Which boroughs restrict which loft conversion type?
Article 4 directions remove some or all permitted development rights in specific conservation areas, meaning even a modest rear dormer that would be permitted development elsewhere needs a full application under the council's adopted Local Plan. Article 4 coverage is significant in parts of the London Borough of Wandsworth, the London Borough of Richmond upon Thames, the London Borough of Hackney, and the London Borough of Southwark, and exists in pockets across most inner London boroughs — the Bedford Park Conservation Area, spanning the border of the London Borough of Ealing and the London Borough of Hounslow, is a well-known example where Article 4 removes Class B rights on principal elevations. The London Borough of Enfield and the London Borough of Redbridge, with more suburban, less densely conservation-designated stock, see a higher proportion of straightforward Class B permitted development schemes.
The only way to know for certain is to check the specific property against the council's adopted Article 4 map, conservation area boundary, and any relevant Supplementary Planning Document (SPD) covering roof extensions — assumptions based on a neighbouring street are not reliable, since Article 4 directions are drawn property by property in some boroughs. Where a scheme needs full planning permission rather than permitted development, the 2025 householder application fee is £528, payable to the council at submission alongside the drawing set.
What headroom and stair rules decide which type will actually work?
Building Regulations 2010 Approved Document K (Part K) sets stair requirements that constrain loft design regardless of external planning route: maximum pitch of 42 degrees, and minimum headroom of 1.9m over the full width of the stair, reducing to 1.8m at the side of the stair where the ceiling slopes. A roof with a shallow existing pitch may not give enough ridge height for a Velux conversion to clear this headroom test at all, which is often the deciding factor in specifying a dormer instead — the dormer's near-vertical wall creates the headroom a sloped rooflight conversion can't.
Fire safety is the other technical driver: Approved Document B (Part B) requires a third storey (which a loft conversion typically creates) to have a protected stairway with 30-minute fire resistance to walls and doors enclosing the escape route, plus mains-wired, interlinked smoke alarms on every storey. This applies to every loft type equally and has to be resolved in the Building Regulations drawing set regardless of whether the roof form outside is a dormer, hip-to-gable, or mansard.
What changes in the drawing set between the different loft types?
The planning drawings differ mainly in roof form and elevations, but the Building Regulations and structural package differs more substantially:
- Velux: minimal structural intervention — usually just floor joist strengthening calculations and rooflight structural openings.
- Dormer: new steel or timber dormer structure calculations, flashing and weathering details, plus floor and stair structural calcs.
- Hip-to-gable: new gable wall structural design, tie-in to existing party wall or external wall, plus roof structure calculations for the new gable end.
- Mansard: most extensive — near-full new roof structure calculations, often requiring temporary roof support design during construction, and typically the most detailed Party Wall etc. Act 1996 consideration since mansards frequently touch shared party walls along a terrace.
Where the loft conversion is adjacent to a shared wall, the Party Wall etc. Act 1996 requires two months' notice for a party structure notice and one month for a line of junction notice before work starts, regardless of which type is chosen — this is a timing item to build into the programme early, not something to leave until drawings are finished.
Can you combine loft conversion types on the same roof?
Yes, and it's common practice on London semi-detached and detached houses: a hip-to-gable conversion squaring off the side roof is very often paired with a rear dormer in the same scheme, because together they maximise usable floor area across the full width of the loft rather than leaving the hipped side as unconverted roof space. Where this combination is being designed under permitted development, both additions count toward the same single 40m³ (terrace) or 50m³ (semi-detached/detached) Class B volume allowance — they are not separate budgets — so the structural and volume calculations for a combined scheme have to treat the whole addition as one figure, checked against the total permitted development allowance in a single calculation, not two.
Velux rooflights are also frequently added alongside a dormer scheme, typically over a rear slope that isn't extended, to bring additional natural light into secondary rooms such as a bathroom or a smaller bedroom without adding to the overall roof volume calculation, since rooflights fitted flush to an unaltered roof slope don't count as an increase in roof volume under Class B.
What does the drawing process for a loft conversion typically look like?
Regardless of which type suits the roof, the drawing process generally follows the same sequence. First, a measured survey of the existing roof establishes actual ridge height, pitch, and headroom — critical because published GPDO volume limits and Approved Document K headroom rules are only useful once measured against the real roof, not an assumed standard one. Second, concept drawings test stair position and headroom against Approved Document K's 42-degree pitch and 1.9m/1.8m headroom requirements, since a stair that doesn't fit makes the whole scheme unviable regardless of roof type. Third, the planning route is confirmed — Lawful Development Certificate under s.192 for permitted development, or a full householder application where Class B doesn't apply — and the relevant drawing set produced. Finally, Building Regulations drawings and structural calculations for the confirmed design are issued, covering fire safety under Approved Document B, insulation under Approved Document L, and the structural calculations for whichever roof type has been chosen.
Because the roof type decision cascades through every later stage — planning route, structural calculations, fire strategy — it's worth resolving at concept stage with a chartered technologist rather than defaulting to whichever type a builder suggests first, since the wrong type chosen early can mean a full planning application and redesign later where a correctly assessed Class B scheme would have avoided both.
What does Part L insulation compliance add to a loft conversion drawing set?
Approved Document L (Part L) sets the insulation and energy-efficiency standards a loft conversion must meet, covering roof U-values, window specification, and airtightness detailing — this is drawn into the Building Regulations package alongside the Part B fire strategy and Part K stair design, not treated as a separate submission. Building Control charges for checking a typical loft conversion drawing set against Part A (structure), Part B, Part K and Part L together generally run from £500 to £1,000 depending on the council and whether a full plans or building notice route is used, and Building Control sign-off is usually confirmed within 10 working days of the final inspection once construction is complete.
Where the roof form is a mansard rather than a smaller dormer, the extent of new external wall and roof area subject to Part L compliance is proportionally larger, which is one reason mansard conversions typically sit at the £65,000–£90,000+ end of the cost range referenced earlier rather than the £45,000–£65,000 typical of a dormer.
Frequently asked questions
What is the difference between a dormer and a mansard loft conversion?
A dormer adds a discrete box-shaped extension to a section of roof slope, usually within GPDO permitted development volume limits. A mansard replaces most of the roof slope with a steep, near-vertical 72-degree face, giving the most added volume but usually needing full planning permission rather than permitted development.
Can I do a hip-to-gable loft conversion on a terraced house?
No. Hip-to-gable only applies to semi-detached or detached houses with a hipped (sloped) side roof, since mid-terrace houses share party walls where there's no hip to convert.
How much roof volume can I add without planning permission?
Under GPDO 2015 Schedule 2 Part 1 Class B, a terraced house can add up to 40m³ and a semi-detached or detached house up to 50m³ as permitted development, subject to conditions on materials, no verandas or balconies, and obscure glazing on side-facing windows.
Does permitted development apply to flats or conservation areas?
Class B permitted development rights do not apply to flats or maisonettes, and do not cover the principal elevation of a house within a conservation area. A full householder planning application is needed instead in those cases.
What headroom do I need for a loft conversion stair?
Approved Document K requires a maximum stair pitch of 42 degrees and minimum headroom of 1.9m over the stair, reducing to 1.8m at the side where the ceiling slopes. This often rules out a Velux-only conversion on shallow-pitched roofs.
Do I need a fire-rated stairway for a loft conversion?
Yes, in almost every case. Approved Document B requires a protected stairway with 30-minute fire resistance and interlinked smoke alarms on every storey once a loft conversion creates a third storey in the house.
How long does the council take to decide a loft conversion planning application?
A standard householder planning application must be determined within 8 weeks of validation. Lawful Development Certificate applications confirming permitted development status typically also take around 8 weeks in practice.
Do I need to notify neighbours under the Party Wall Act for a loft conversion?
If work involves a shared party wall — common with dormers and mansards on terraced and semi-detached houses — the Party Wall etc. Act 1996 requires two months' notice for a party structure notice and one month for a line of junction notice before work begins.
Can I combine a hip-to-gable conversion with a rear dormer?
Yes, this is a common combination on semi-detached and detached houses with hipped roofs. Both additions count toward the same single 40m³ or 50m³ Class B volume allowance, so they must be calculated together as one total, not as separate budgets.
Does a loft conversion need to meet Part L insulation standards?
Yes. Approved Document L (Part L) sets roof U-value, window and airtightness standards that apply to every loft conversion type and are included in the Building Regulations drawing set alongside Part A, Part B and Part K.
