The permitted development rules for two-storey extensions
A two-storey rear extension is permitted development only within tight limits: no more than 3 m deep, at least 7 m from the rear boundary, no higher than the existing house, eaves matching the existing eaves, materials similar in appearance, and any upper-floor side window obscure-glazed and non-opening below 1.7 m.
Most two-storey extensions exceed one of those limits, most often the 7 m rear boundary rule on London plots, so the usual route is a householder planning application. Flats have no permitted development rights.
What changes in Hemel Hempstead
Extensions of more than one storey are not permitted development in a conservation area, so planning permission is always needed there.
The local constraints that most often decide the route in Hemel Hempstead are listed below, taken from our Hemel Hempstead area page. Each one is a reason to confirm the address on Dacorum's constraints map before relying on permitted development.
- Conservation areas. Dacorum's conservation areas include Hemel Hempstead Old Town, Boxmoor, Berkhamsted, Tring and several villages, with a high concentration of listed buildings in the Old Town High Street, Berkhamsted and Tring. Roof extensions are not permitted development inside them, and officers apply the character appraisal to extensions and dormers. Check the council's constraints map for the exact boundaries before relying on permitted development. Source
- Green Belt. Green Belt surrounds Hemel Hempstead, Berkhamsted, Tring and Kings Langley and covers most of the countryside between them, including the edges of Leverstock Green, Woodhall Farm and Bovingdon. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, and outbuildings face a stricter test on size and siting. Check the constraints map for the boundary at your address. Source
- Chilterns National Landscape. The Chilterns National Landscape (formerly the Chilterns AONB) covers the west of the district around Berkhamsted, Tring and the Bulbourne valley. Extensions and outbuildings there are assessed for their effect on the landscape, and larger home extension prior approval is more limited. Check the constraints map before relying on permitted development. Source
- Grand Union Canal, River Gade and River Bulbourne flood risk. Land alongside the Grand Union Canal, the River Gade through the Old Town, Gadebridge and Apsley, and the River Bulbourne through Boxmoor and Berkhamsted 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 Old Town High Street, Boxmoor, Berkhamsted, Tring and the village cores contain many listed houses, inns and churches. Listed building consent is needed for almost any alteration, internal or external. Source
What Dacorum planning officers look for
Dacorum officers assess householder applications against the Core Strategy 2013, the Site Allocations 2017 and the council's supplementary guidance on residential extensions, with the emerging Local Plan gaining weight as it advances. On the new-town terraces they tend to look for rear extensions that respect the pattern of the row, pass the 45-degree test to the neighbour's nearest habitable window and keep parking and the shared landscaping of Radburn layouts intact; on the inter-war semis of Leverstock Green and Boxmoor they expect side extensions set back from the front wall and below the ridge with a gap kept to the boundary. In the Old Town, Boxmoor, Berkhamsted and Tring conservation areas they expect traditional materials and subordinate dormers on rear slopes only. In the Green Belt they check the extension against the original house and resist large outbuildings. Common refusal reasons are full-width box dormers, side extensions that create a terracing effect, loss of parking and disproportionate Green Belt extensions. Outside conservation areas, where permitted development rights have not been removed, rear-dormer lofts within 40 m³ on a terrace or 50 m³ on a semi are routinely lawful and we secure a Lawful Development Certificate as proof. Dacorum 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.
