The permitted development rules for side extensions
A single-storey side extension is permitted development on a house if it is no wider than half the width of the original house, no higher than 4 m, with eaves no higher than 3 m where it comes within 2 m of a boundary, and finished in materials similar in appearance to the house.
Anything wider than half the original house, any two-storey side extension, and any side extension on a flat needs planning permission. Garage conversions that keep the walls are usually permitted development; extending beyond them is assessed as a side extension.
What changes in Watford
Side extensions are not permitted development in a conservation area at all, so there a planning application is always required.
The local constraints that most often decide the route in Watford are listed below, taken from our Watford area page. Each one is a reason to confirm the address on Watford's constraints map before relying on permitted development.
- Conservation areas. Watford's conservation areas include Cassiobury, Nascot, Estcourt and Oxhey Village, as well as parts of the town centre and High Street. Roof extensions are not permitted development inside them, and officers apply the character appraisal to extensions, dormers and front alterations. Check the council's constraints map for the exact boundaries before relying on permitted development. Source
- Article 4 direction on HMOs. We understand Watford Borough Council has used an Article 4 direction to remove permitted development for converting a house into a small house in multiple occupation (C3 to C4), so HMO conversions normally need planning permission. Check the constraints map for the current extent of the direction and for any further directions affecting conservation areas. Source
- Cassiobury Park and Whippendell Woods edges. Houses backing onto Cassiobury Park, the Grand Union Canal corridor and the edge of Whippendell Woods sit against protected open space, often with tree preservation orders and some Green Belt land. Officers look closely at rear extensions and outbuildings visible from the park, and work near protected trees needs an arboricultural report. Check the constraints map for Green Belt and TPO boundaries. Source
- River Colne and Grand Union Canal flood risk. Land alongside the River Colne and the Grand Union Canal on the west and south of the town, including parts of Cassiobury, Holywell and Oxhey, sits in Flood Zones 2 and 3. Ground-floor extensions and basements there may need a flood risk statement with the application. Source
- Listed buildings. The High Street, St Mary's churchyard, Nascot and the Cassiobury area contain listed houses, almshouses and churches. Listed building consent is needed for almost any alteration, internal or external. Source
What Watford planning officers look for
Watford officers assess householder applications against the Watford Local Plan 2021–2038 and the council's residential design guidance. On the Edwardian and inter-war semis that make up much of the town they tend to look for two-storey side extensions set back from the front wall and below the main ridge, with a gap kept to the side boundary so the pair does not read as a terrace; rear extensions that pass the 45-degree test to the neighbour's nearest habitable window; and dormers that are subordinate, set back from the eaves and clad in matching tile. In Cassiobury and Nascot they expect extensions to follow the appraisal and keep the spacious plot pattern. Common refusal reasons are full-width box dormers, side extensions that create a terracing effect and front dormers in conservation areas. Outside conservation areas, hip-to-gable and rear-dormer lofts within 50 m³ on a semi are routinely lawful and we secure a Lawful Development Certificate as proof. Watford offers a paid pre-application service. Decisions can run past the statutory 8 weeks; allow 8–12 weeks.
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.
