The permitted development rules for loft conversions
A loft conversion with a rear dormer is permitted development on a house if the added roof volume is no more than 40 m³ on a terraced house or 50 m³ on a semi-detached or detached house, the dormer sits at least 200 mm back from the eaves and no higher than the ridge, there is no dormer on the roof slope facing the road, materials are similar in appearance, and side windows are obscure-glazed.
Flats and maisonettes have no permitted development rights, so a loft to a top-floor flat always needs planning permission. Full-width box dormers that reach the ridge, and hip-to-gable conversions on semis whose volume exceeds 50 m³, also fall outside the allowance.
What changes in Islington
Roof extensions are not permitted development in a conservation area, so a dormer or hip-to-gable there needs a householder planning application designed to the area's appraisal.
The local constraints that most often decide the route in Islington are listed below, taken from our Islington area page. Each one is a reason to confirm the address on Islington's constraints map before relying on permitted development.
- Conservation areas. Around 40 conservation areas cover roughly half of Islington, including Barnsbury, Canonbury, Highbury Fields, Clerkenwell Green, Tufnell Park and Hillrise. Roof extensions and cladding in these areas need planning permission rather than permitted development. Check the council's constraints map for your address before assuming PD applies. Source
- Article 4 directions. Islington has a Class MA Article 4 direction covering the Central Activities Zone and its town centres, removing the commercial-to-residential prior approval route there, and Article 4 directions in several conservation areas removing permitted development for front alterations. We confirm coverage for your address with the council. Source
- Listed buildings. Barnsbury, Canonbury, Clerkenwell and the Georgian squares contain many listed terraces. Listed houses need listed building consent for almost any alteration, internal or external, and extensions to neighbours are judged on their effect on the setting of the listed terrace.
What Islington planning officers look for
Islington is one of the stricter London authorities on householder design and applies the Urban Design Guide closely. Rear extensions are generally expected to be single storey and no deeper than 3 m on terraces, side returns must keep parapets low against the neighbour's garden and roof extensions in conservation areas are frequently refused unless they follow an established pattern on the terrace. Mansards are accepted where the terrace already has them; front dormers are not. Pre-application advice is available for a fee and, in our experience, is worth using for anything in Barnsbury or Canonbury and for all basements.
Fees and timelines
The council fee for a householder application is £528 and for a Lawful Development Certificate £264 (England, from 1 April 2025; verify before applying). Both are decided within 8 weeks of validation.
Our own fee for the drawings and the application is fixed once quoted and depends on the project type, its size and the constraints above. Answer six questions in the quote tool and we email you an indicative figure within minutes, then a written fixed-fee quote within 24 hours.
