Most loft conversions in England don’t need planning permission — they’re covered by permitted development rights, meaning £0 in planning fees. If yours falls outside those limits, the fee is the same flat £548 householder application rate that applies to extensions.
The question that actually matters here isn’t “how much does the fee cost.” It’s “does my specific conversion type, at my specific volume, on my specific house, still qualify.” That’s a more precise test than most guides let on.
👉 Use the calculator to confirm your exact fee
The volume limit that decides everything
Under permitted development (GPDO Schedule 2, Part 1, Class B), you can add:
- 40 cubic metres of extra roof volume on a terraced house
- 50 cubic metres on a semi-detached or detached house
This is a cumulative allowance tied to the property, not a fresh allowance per project. If a previous owner already used part of it — even a small dormer added a decade ago — what’s left is reduced accordingly, and that history doesn’t reset when you buy the house. It’s worth checking the property’s planning history with the local council before assuming the full 40m³ or 50m³ is available to you.
Why one dormer type needs permission and another doesn’t
This is the distinction most competitor guides blur together:
- Rear dormer — the most common conversion type, and usually fine under permitted development, provided it’s set back at least 20cm from the original eaves and stays within the volume cap.
- Hip-to-gable — reshapes a hip roof into a vertical gable wall, effectively squaring off one side. Generally permitted development if within volume limits, since it doesn’t project outward the way a dormer does — it reshapes rather than adds a box.
- Mansard — alters the roof into a near-vertical wall topped with a flat roof, close to a full extra storey in appearance. This almost always needs a full planning application, because it changes the roof’s silhouette more substantially than the volume calculation alone captures.
- Rooflights only — no structural roof enlargement, just windows set into the existing roof plane. Typically doesn’t need permission at all, and doesn’t even count against your volume allowance, since nothing is being added.
Two houses on the same street doing “a loft conversion” can land on completely different outcomes — £0 versus £548 — purely based on which of these four categories the specific design falls into, independent of the room’s final size inside.
A detail worth flagging: outrigger and semi-detached corner plots
Houses with a rear outrigger (an extension at the back, common in Victorian terraces) sometimes already have roof volume “used up” by a previous single-storey rear addition being counted differently than expected — worth having a surveyor or architect confirm the precise baseline volume for your specific roof shape before assuming a standard calculation applies. Corner plots facing two roads can also have two “principal elevations” in a council’s eyes, restricting where a dormer can sit more than a standard mid-terrace plot.
Other conditions that trigger the full fee
Even within the volume limit, permitted development is lost if:
- The conversion is higher than the existing roof ridge
- A dormer faces the principal elevation (the side facing the road)
- Materials don’t reasonably match the existing house’s external appearance
- The property is a flat, not a house — flats never carry these permitted development rights
- The property sits in a Conservation Area, National Park, AONB, or is listed — dormers usually need permission here even within volume limits, though rooflights are often still exempt
Worked example
A semi-detached house has 8m³ of its loft allowance already used by a previous small dormer. The owner wants a new rear dormer adding 38m³.
- Total after conversion: 8 + 38 = 46m³
- Limit for semi-detached: 50m³
- Result: within the cap — likely no £548 fee, provided the setback, ridge height, and materials conditions are also met
Had the new dormer added 44m³ instead, the total would reach 52m³ — exceeding the limit and requiring a full £548 application for what might look, from the street, like a very similar-sized addition.
If you’re not certain: Lawful Development Certificate
Because the volume calculation isn’t always obvious from a rough sketch, many homeowners apply for a Lawful Development Certificate to get formal confirmation they didn’t need permission — useful proof for future buyers and mortgage lenders, who often ask for it specifically. This costs roughly half the householder fee, around £274 for a proposed loft conversion.
Building regulations — a separate cost entirely
Even a loft conversion needing no planning permission still needs building regulations approval — structural work, fire escape provision, insulation, and staircase design are all assessed separately. Most enforcement problems with loft conversions come from missing building regs sign-off, not missing planning permission, so don’t treat “no planning fee” as “no further approval needed.”
For the official permitted development rules, see Planning Portal’s guidance on loft conversions.
Quick answer summary
- Within volume limits, correct dormer type, not on designated land: £0 (permitted development)
- Mansard conversion, exceeds volume limit, or on designated land: £548
- Certificate of Lawful Development (optional, for proof): ~£274
- Building regulations approval: separate fee, required regardless
Use the calculator above once you know which category your conversion falls into.