The question isn't really "should I convert my loft?" — it's "which type of loft conversion will actually work for my house?" And that second question is where most homeowners get stuck.
There are four main types — Velux, Dormer, Hip-to-Gable, and Mansard — and they're not interchangeable. The right one depends on your roof structure, your headroom, your budget, and what your local planning authority will allow. Get it wrong and you'll either spend more than you needed to, or end up with a conversion that doesn't give you the space you were hoping for.
We've completed 63 loft conversions across Hertfordshire and North London since 2020. What follows is the same framework we use when we assess a new project — the questions we ask, the trade-offs we weigh, and the regional patterns we've noticed that don't appear in any generic guide.
What Is a Loft Conversion, Really?
A loft conversion is the structural transformation of an existing roof space into a habitable room — one that meets Building Regulations for headroom, insulation, fire safety, and means of escape. That last part matters more than most people realise. It's not just "boarding out the loft." It's creating a legal bedroom, office, or bathroom that will appear on your property's floor plan and affect its value and insurability.
It's worth distinguishing a loft conversion from a loft room. A loft room is an informal storage or hobby space accessed by a loft ladder — it doesn't require planning or Building Regulations approval, but it also doesn't add legal floor area to your home. A loft conversion, by contrast, requires a fixed staircase, proper fire doors, and compliance with Part B (fire), Part F (ventilation), and Part L (energy efficiency) of the Building Regulations. According to the RICS Residential Mortgage Valuation Specification 2024, only converted loft space that meets these standards counts toward the property's habitable floor area for valuation purposes.
The four types differ primarily in how much they alter the external envelope of your roof. Velux conversions don't alter it at all. Dormers extend it outward from the slope. Hip-to-Gable conversions extend it sideways. Mansard conversions rebuild it almost entirely. More alteration generally means more space, more cost, and more planning complexity.
2026 Cost Overview
Figures based on TCM's 63-project dataset (Hertfordshire and North London, 2020–2026, updated July 2026) and Checkatrade's 2025 national cost report. Costs include structural work, insulation, electrics, plasterboarding, and staircase. They exclude furniture, fitted wardrobes, and VAT. BCIS Build Cost Index Q1 2026 uplift of 4.2% applied to 2025 benchmarks.
Velux / Rooflight
The lightest touch
Rear Dormer
The most popular choice
Hip-to-Gable
For hipped-roof properties
Mansard
Maximum space, maximum impact
Value-added estimates based on Nationwide Building Society House Price Index Q1 2026 and Savills Residential Research 2025. Individual results vary by location, specification, and market conditions.
Each Type in Depth
Velux / Rooflight Conversion
A Velux conversion works entirely within your existing roofline. We install roof windows — typically Velux-brand, though other manufacturers make equivalent products — flush with the slope, add a staircase, insulate the floor and roof, and board out the space. The external appearance of your roof doesn't change at all.
That's the appeal. No scaffolding for structural alterations, no planning application in most cases, and a build time of four to six weeks. It's also the cheapest option by a significant margin — our median project cost is £22,500, compared to £46,000 for a rear dormer.
But there's a real constraint: you need existing headroom. Building Regulations require at least 2.2 metres at the highest point for the space to count as habitable. Many older Hertfordshire semis — particularly 1930s and 1950s builds — have enough pitch to meet this. But if your ridge height is marginal, a Velux conversion will leave you with a room that feels cramped at the edges, even if it technically complies. We measure every loft before recommending this type, and we turn down Velux jobs where the headroom won't deliver a comfortable room.
Best for: Homes with ≥2.4 m existing headroom; minimal external change required.
Not ideal for: Low-pitch roofs; anyone needing full standing headroom across the whole floor.
Rear Dormer Conversion
A dormer is a box-shaped structure that projects outward from the slope of your existing roof, usually at the rear. It creates vertical walls and a flat or low-pitched roof within the dormer box — which means full standing headroom across most of the new floor, not just at the ridge.
This is the most popular type we build, and for good reason. It works on almost any property with a pitched roof, it usually falls within Permitted Development rights (so no planning application needed), and it delivers a genuinely usable room — typically 20 to 30 square metres — rather than a space that's only comfortable in the middle.
The cost range is £38,000 to £58,000. The variation comes from the size of the dormer, the specification of the windows, whether you're adding an en-suite bathroom, and the complexity of the staircase. Our median project cost is £46,000 for a rear dormer with a bathroom and a standard staircase.
One thing worth knowing: a rear dormer on a terraced house in Hertsmere or Barnet is almost always Permitted Development. But the same dormer on a property in a conservation area, or on a flat in a converted house, may need full planning permission. We check the specific rules for your address before we quote.
Best for: Terraced and semi-detached homes; en-suite bedroom; home office.
Not ideal for: Front-facing elevations in conservation areas without planning consent.
Hip-to-Gable Conversion
If your property has a hipped roof — where the sides slope inward as well as the front and back — then a Hip-to-Gable conversion is often the answer. We extend the sloping side (the "hip") outward to form a vertical gable wall, which squares off the roof and dramatically increases the internal volume.
On its own, a Hip-to-Gable conversion adds space but not necessarily headroom across the full width. That's why it's almost always combined with a rear dormer — the two together give you a large, full-height loft room that runs the entire width of the property. This combination is sometimes called an L-shaped dormer, and it's the most common type we build on detached and end-of-terrace properties in Radlett, Potters Bar, and Stanmore.
The planning position is more complex than for a standard dormer. A Hip-to-Gable conversion alters the side elevation of your property, which means it often falls outside standard Permitted Development criteria — particularly on semi-detached homes where the side elevation faces a neighbour. We've found that Hertsmere Borough Council and Barnet Council both take a consistent approach here: Hip-to-Gable on a detached property is almost always approved; on a semi-detached, it depends on the street scene and whether neighbours have done the same.
Best for: Detached and end-of-terrace with hipped roof; often combined with rear dormer.
Not ideal for: Mid-terrace properties (no hip to convert).
Mansard Conversion
A Mansard conversion rebuilds the entire rear roof slope. The new structure has a near-vertical back wall (typically at 72 degrees from horizontal) and a shallow flat roof on top. The result is a full-width, full-height loft room — effectively a new storey — that runs the entire depth of the property.
It's the most expensive type, and it almost always requires planning permission because the change to the external appearance is significant. But for Victorian and Edwardian terraces — which are common across Barnet, Harrow, and parts of Watford — it's often the only way to get a genuinely spacious loft conversion. The existing roof pitch on these properties is frequently too shallow for a dormer to deliver adequate headroom, and there's no hip to convert. A Mansard solves both problems.
The BCIS Build Cost Index Q1 2026 shows structural costs up 4.2% year-on-year, which has pushed Mansard projects toward the upper end of the range. Our median project cost is £68,000, including planning fees, structural engineering, and a bathroom. That's a significant investment — but on a four-bedroom Victorian terrace in Barnet, a well-executed Mansard conversion can add £80,000 to £120,000 to the property's value, according to Savills Residential Research 2025.
Best for: Victorian and Edwardian terraces; urban properties; anyone wanting a full extra storey.
Not ideal for: Budget-conscious projects; properties where planning is unlikely to be granted.
Side-by-Side Comparison
| Feature | Velux | Dormer | Hip-to-Gable | Mansard |
|---|---|---|---|---|
| Cost range (2026) | £18k–£28k | £38k–£58k | £42k–£65k | £55k–£85k |
| TCM median | £23,500 | £48,000 | £54,500 | £71,000 |
| Space gained | Existing footprint | 20–30 m² | 25–40 m² | 30–50 m² |
| Full standing headroom | Ridge only | Throughout dormer | Throughout | Throughout |
| Permitted Development | ✓ Usually | ✓ Usually | ⚠ Often needs PP | ✗ Always needs PP |
| Build time | 4–6 weeks | 8–12 weeks | 8–12 weeks | 12–16 weeks |
| Value added | 10–12% | 15–20% | 15–20% | 20–25% |
| Best property type | Any with headroom | Terraced / semi | Detached / end-terrace | Victorian / Edwardian terrace |
Planning Permission and Building Regulations
The planning rules for loft conversions are set out in Schedule 2, Part 1, Class B of the Town and Country Planning (General Permitted Development) (England) Order 2015 (the GPDO). Under Permitted Development, you can add up to 40 cubic metres of new roof space on a terraced house, or 50 cubic metres on a detached or semi-detached property, without a planning application — provided the conversion doesn't alter the principal elevation, doesn't exceed the height of the existing roof ridge, and uses materials that match the existing roof.
Velux and rear dormer conversions typically fall within these limits. Hip-to-Gable and Mansard conversions often don't, because they alter the side or rear elevation in ways that exceed the PD criteria. If you're in a conservation area, an Article 4 Direction area, or a listed building, PD rights are restricted or removed entirely — planning permission is required regardless of type.
Even when planning isn't needed, Building Regulations approval is always required. The relevant Parts are: Part A (structure), Part B (fire safety — including a 30-minute fire-resistant ceiling and smoke alarms on every floor), Part F (ventilation), Part K (stairs), and Part L (thermal performance). Your local authority's Building Control team or an Approved Inspector will inspect the work at key stages and issue a Completion Certificate when it's done. That certificate is what your solicitor will ask for when you come to sell.
We handle all Building Regulations applications and inspections as part of our standard service. We don't hand you a set of drawings and leave you to manage the process.
The Things That Catch People Out
Headroom that looks fine but isn't
A lot of homeowners measure the ridge height and assume they're fine. But the Building Regulations requirement is 2.2 m at the highest point — and the usable floor area is only the part where you can stand upright. A loft with 2.3 m at the ridge but a steep pitch can end up with a very small usable area. We measure the full headroom profile before recommending a type.
The staircase you didn't plan for
A loft conversion needs a fixed staircase, not a loft ladder. That staircase has to comply with Part K (minimum 42-degree pitch, minimum 220 mm going, minimum 196 mm rise). Finding space for it on the floor below is often the hardest part of the design — especially in terraced houses where the landing is narrow.
Party wall notices on dormer and Mansard jobs
If your loft conversion involves work on or near a shared wall — which is almost always the case on terraced and semi-detached properties — you'll need to serve a Party Wall Notice under the Party Wall etc. Act 1996. This has to happen at least two months before work starts. We flag this at the survey stage, but it's worth knowing upfront. See our <Link to='/blog/what-is-a-party-wall-agreement'>Party Wall Agreement guide</Link> for the full process.
Structural engineer fees that weren't in the quote
Dormer and Mansard conversions require structural calculations — for the new steel beams, the altered roof structure, and the floor joists. Some builders quote without these. We include structural engineering in our fixed-price quotes, so there are no surprises.
Conservation area restrictions you didn't know about
Borehamwood, Radlett, Barnet, and parts of Watford all have conservation areas where PD rights are restricted. A rear dormer that would be straightforward elsewhere may need planning permission — and may not be approved at all if the street scene is protected. We check the planning constraints for your specific address before we quote.
The insulation that affects your room height
Part L requires a minimum U-value of 0.18 W/m²K for the roof. Achieving this with rigid PIR insulation (the most space-efficient option) requires approximately 120–140 mm of board. That insulation goes between and below the rafters — which means it reduces your usable headroom by up to 150 mm. On a marginal loft, this can be the difference between a comfortable room and one that doesn't work.
Our Regional Recommendation Pattern
After 63 loft conversions across Hertfordshire and North London (updated July 2026), we've noticed that the "right" type varies significantly by postcode — not just by property type. Here's what we actually recommend in each area, and why.
Borehamwood & Elstree (WD6)
Rear dormer on 1950s–1970s semis; Velux on 1990s detacheds with adequate pitch. Hip-to-Gable rarely needed — most properties have gable ends already.
Hertsmere Borough Council is generally supportive of rear dormers on non-conservation streets.
Radlett & Shenley (WD7)
Hip-to-Gable + rear dormer combination on the large detacheds. Velux on bungalows with high pitch. Mansard rarely appropriate given the property types.
Radlett Conservation Area covers the village centre — check your specific address before assuming PD.
Barnet & New Barnet (EN5, EN4)
Mansard on Victorian terraces (the most common property type). Rear dormer on 1930s semis. Hip-to-Gable on detacheds in East Barnet.
Barnet Council has several Article 4 Direction areas. We check every address individually.
Harrow & Stanmore (HA1–HA3, HA7)
Rear dormer is the default on semis. Mansard increasingly popular on the Victorian terraces around Harrow-on-the-Hill. Hip-to-Gable on detacheds in Stanmore.
Harrow Council is one of the more active planning authorities in the area — we've found that pre-application advice is worth getting on any Mansard job.
Watford & Bushey (WD17–WD25)
Rear dormer on the large stock of 1930s semis. Velux on properties with high-pitch roofs in Bushey. Mansard rare — the property types don't typically suit it.
Three Rivers and Watford Borough Councils both have straightforward PD policies for standard rear dormers.
St Albans & Harpenden (AL1–AL5)
Rear dormer on semis and terraces. Hip-to-Gable on the large detacheds in Harpenden. Conservation area coverage is significant — planning applications more common here than elsewhere.
St Albans City and District Council has extensive conservation area coverage. We always check before quoting.
How TCM Approaches a Loft Conversion Project
We start every loft project with a free survey. Not a sales visit — an actual structural assessment. We measure the headroom profile, check the roof timbers, identify any party wall obligations, and confirm the planning position for your specific address. That takes about an hour, and it's the only way to give you a quote that won't change when we open up the roof.
We then produce a fixed-price quote that includes structural engineering, Building Regulations applications, and all labour and materials. No provisional sums, no "subject to survey" caveats. If we quote it, we build it for that price.
Our loft conversion team has been working together for over eight years. The same site manager who surveys your loft will be on site every day of the build. We don't subcontract the structural work or the roofing — those are done by our own directly employed team.
For more on how we compare to other options, see our Loft Conversion vs House Extension comparison and our full cost comparison guide.
Related Guides
Loft Conversions Service
Our full loft conversion service — what's included, how we work, and how to get a quote.
Loft vs Extension: Full Cost Comparison
Side-by-side cost breakdown with TCM's 63-project dataset.
Loft Conversion vs House Extension
Which adds more value? Which is right for your situation?
Permitted Development Rights Guide
The full PD rules for loft conversions, extensions, and outbuildings.
Party Wall Agreements Explained
When you need one, how to serve notice, and what it costs.
House Extension Cost Guide 2026
Verified cost data for single-storey, double-storey, and wrap-around extensions.
Flat Roof Replacement Options
GRP, EPDM, felt, and liquid-applied systems compared — lifespan, cost, and best use.
Not sure which type suits your home?
We'll survey your loft for free, tell you which type will work, and give you a fixed-price quote — no obligation, no sales pressure.