Loft conversion drawings for a Swanley house cost £645 drawings + £150 survey (£795 total) with us when the dormer needs a planning application, survey to submission. If the dormer falls within permitted development, the route is a certificate of lawfulness at from £545 drawings + £150 survey (£695 total), which proves it is lawful before your builder starts. Either way the council's charge is separate and shown on your quote.
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Which of the two you need is the first thing we settle, and in Swanley it usually comes down to the shape of the roof and where the house sits.
Do I need planning permission for a dormer in Swanley?
Often not. Much of Swanley's housing is twentieth-century semi-detached and terraced houses with hipped or gabled roofs, the type most lofts are built into. A rear dormer on such a house is permitted development if it adds no more than 50 cubic metres of roof space to a semi-detached house, or 40 cubic metres to a terrace, does not face the road, sits below the ridge and uses similar materials (MHCLG technical guidance).
On a hipped-roof semi, a hip-to-gable change plus a full-width rear dormer often comes close to the 50 cubic metre limit, so the volume calculation decides it. We calculate it from the measured survey, not from an estate agent's plan, and show the figure on the drawings. Our loft permitted development guide covers the other conditions, such as side windows being obscure-glazed.
Three things take a Swanley loft out of permitted development:
- the house is a flat or maisonette, which has no householder rights;
- an earlier permission removed the rights by condition, which we check in the planning history;
- the house is in a conservation area, where roof extensions are not permitted development.
Is Swanley in the Green Belt?
The town itself is a built-up area, but the countryside around it is Green Belt, as is 93% of Sevenoaks District (Sevenoaks Residential Extensions SPD, para 3.4). Houses on the edges of town, in Swanley Village and out towards Crockenhill and Hextable may be inside it.
Green Belt status does not by itself remove the loft permitted development right. It matters when you need planning permission: the council then asks whether the dormer, together with any earlier additions, is a disproportionate addition to the original house. A large box dormer that would pass in the town can fail that test on a cottage outside it. In the Green Belt, Sevenoaks' checklist also asks for plans and elevations of the whole house (Sevenoaks validation checklist).
What does Sevenoaks want from a dormer that needs permission?
The council's Residential Extensions SPD sets its dormer rules plainly. Loft extensions should be in proportion to the roof, lined up with the doors and windows below, kept under the ridge and set back at least 20 centimetres from the eaves and the sides (SPD para 4.35). New front dormers are not normally allowed in streets that have none, and rooflights are the preferred alternative at the front (para 4.34). Large dormers that are disproportionate to the house, or that overlook a neighbour unacceptably, will not be allowed (paras 4.36 and 4.37).
In practice that means a Swanley semi that needs permission usually gets a rear dormer held in from the party wall and the hip, with rooflights to the front slope. We draw that version first and show you how much floor area it gives before you decide.
What about Swanley Village conservation area?
Swanley Village, the old settlement north-east of the town, is one of Sevenoaks District's 41 conservation areas (Sevenoaks conservation areas). There, any dormer needs a planning application, and the design has to preserve or enhance the area's character. The checklist may also call for a preliminary ecology appraisal when the roof of an older or rural building is altered, because bats roost in old roofs. We confirm at the quote stage whether either applies.
What does the whole loft cost to get approved?
The council's charge depends on the route, and it is paid to Sevenoaks District Council, not to us:
- Permitted development with a certificate. Our certificate of lawfulness service is from £545 drawings + £150 survey (£695 total), and the council's fee for a certificate for a proposed use is £274.
- Planning application. Our service is £645 drawings + £150 survey (£795 total), and the householder application fee is £548.
- No paperwork at all. You can build a permitted development dormer without a certificate. It is legal, but you carry the risk if a buyer's solicitor or the council later disputes the volume.
The choice between the first two is usually made by the volume figure. A Swanley semi whose dormer and hip-to-gable come in at, say, 46 cubic metres can go the certificate route; one at 55 cubic metres needs either a smaller dormer or a planning application, and in the town that application is judged against the dormer rules above rather than the Green Belt test. We show you both versions, with the floor area each gives, before you pick. That comparison is part of the design stage, not an extra.
One Swanley-specific point: if a previous owner already built a rear extension or a converted garage, that does not use up the loft's 50 cubic metres, which is a separate allowance for the roof. An earlier roof extension does count, though, and we check for one in the planning history.
What happens after the drawings?
Every loft conversion needs building regulations approval, whatever the planning route: fire escape, stair head height, insulation and structure. Our building regulations drawings and structural calculations cover that, and our fire safety guide for lofts explains the escape rules. If the dormer sits on a party wall, a party wall notice is usually needed.
On timing, Sevenoaks decided 463 of its 491 householder applications within 8 weeks between July 2025 and June 2026 (MHCLG live table PS2).
More about the council is on our Sevenoaks page and Kent page, and the service is on our loft conversions page. For extensions in the south of the district, see extension plans in Sevenoaks; for the neighbouring borough, see planning drawings in Dartford.
Frequently asked questions
Is a certificate of lawfulness worth it if my dormer is permitted development? Usually yes. It costs from £545 drawings + £150 survey (£695 total) with us, and it gives you council-issued proof when you sell or when building control asks.
Can I have a dormer at the front of my Swanley house? Not as permitted development, and Sevenoaks will not normally allow a new front dormer in a street without any. Rooflights at the front are the usual answer.
My house is near Swanley but outside the town. Does that change anything? It may. If the house is in the Green Belt, a dormer that needs permission is judged against the size of the original house, so a modest design matters more.
How do you work out the 50 cubic metres? From the measured survey of the roof. We calculate the volume of each new roof element and show the total on the drawings.
If the application goes through the Planning Portal, the Portal adds its own service charge: nothing, £99 or £149 including VAT, depending on the application type and complexity, paid to the Portal at submission and not normally refunded with a council fee (Ashford Borough Council, Planning Portal service charges, 11 September 2026). It is not ours and not the council's. Other submission routes exist; where your council accepts one without a charge, we can use it.
