Planning · Loft conversions
Loft conversions and permitted development
Volume allowances of 40m³ for a terrace and 50m³ for a semi or detached house — cumulative across the building's life, not per owner.
The volume allowance
Most loft conversions are permitted development, subject to a volume allowance measured against the original roof space: 40 cubic metres on a terraced house, and 50 cubic metres on a semi-detached or detached house.
That allowance is cumulative across the life of the building, not per owner. If a previous owner already built a dormer, part or all of the allowance may already be spent. Establish this before anyone draws anything, because it changes whether you are looking at permitted development or a full application.
Design constraints that come with it
A dormer must be set back at least 20 centimetres from the original eaves, and no part of the extension may sit forward of the roof plane on the principal elevation — which in practice means no front dormers under permitted development.
Materials must be similar in appearance to the existing house, and side-facing windows must be obscure-glazed and non-opening below 1.7 metres from the floor.
Verandas, balconies and raised platforms are excluded. A Juliet balcony behind a window opening is usually acceptable; a projecting balcony is not.
Fire safety drives the design
Part B of the Building Regulations shapes a loft conversion more than most people expect. Adding a second floor to a two-storey house creates a three-storey house, which requires a protected escape route from the top floor to the front door.
In practice that means fire doors to every habitable room off the staircase, an enclosed stairwell, and mains-wired interlinked smoke alarms on every level. Where a protected stair is genuinely impossible, a sprinkler system is sometimes accepted as an alternative — but that is a building control conversation, not an assumption.
Part K governs the staircase itself: minimum headroom of 2 metres over the pitch line (1.9 metres at the centre is acceptable in a loft), and limits on rise and going. This is often the single hardest thing to design into an existing house.
The structure underneath
Existing ceiling joists were sized to hold up a ceiling, not to carry a floor and its occupants. New floor joists are installed alongside them, usually bearing onto steel beams that transfer load to the party walls or an internal structural wall.
All of it is sized by a structural engineer and inspected by building control. Where the steels bear on a party wall, the work is notifiable under the Party Wall etc. Act 1996.
This guide describes national rules for England and was written to be accurate at the time of publication. Local designations can restrict them further, and legislation changes. Always confirm the position for your own address with your local planning authority, or ask us to do it in writing.
Related service: Loft Conversions in London
Answers
Questions we get asked alongside this
Do I need planning permission for loft conversions in London?
It depends on the property type and what is proposed. Many single-storey rear extensions and loft conversions fall under permitted development, but those rights do not apply to flats and are restricted in conservation areas or where an Article 4 direction applies. We check the planning position for your specific address before quoting, and tell you in writing which route applies.
How do you price the work?
From a site survey, written up as a fixed and itemised scope before any deposit is discussed. If something genuinely unforeseen appears once work is opened up, it is priced and agreed in writing before we continue — never added to the final invoice after the fact.
When do I pay?
In arrears of work completed. You pay for work that has been done and inspected, not for work that has been promised. This has been how the company operates since 2008.
Do you handle Party Wall notices?
Yes, where the work requires them. Under the Party Wall etc. Act 1996, notifiable work on a shared wall needs 2 months written notice to the adjoining owner, while excavation near a neighbouring structure and building at the line of junction need 1 month. We serve the notices and manage the award process as part of the job.
Will I get certificates at the end?
Yes. Electrical work is tested and certificated under Part P, and building control issues a completion certificate where an application was required. You receive the handover pack — it is what a buyer's solicitor will ask for when you eventually sell.
How long will it take?
It depends entirely on scope, and we give a programme with the written quote rather than a guess on the phone. What we will not do is start a job we cannot resource, which is the usual reason a build stalls halfway through.
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