Many single-storey rear extensions and most rear dormer loft conversions do not need a full planning application, because they fall under permitted development. But the limits are precise, some homes have lost those rights altogether, and a certificate proving the work was allowed is worth having when you sell.
This guide covers the rules for houses in England as they apply in Dacorum, which includes Hemel Hempstead, Berkhamsted, Tring and Kings Langley. It is general guidance. We check the position for your address on the first site visit.
What permitted development means
Permitted development rights let you extend a house without applying for planning permission, provided you stay inside national limits. They apply to houses only. Flats and maisonettes do not have them, so any extension to a flat needs a full application.
The limits are measured from the original house, meaning the house as it was first built, or as it stood on 1 July 1948 if it is older. Extensions added by previous owners count against your allowance, even if you did not build them.
Single-storey rear extensions
- Up to 3 metres deep for a semi-detached or terraced house, or 4 metres for a detached house.
- No higher than 4 metres overall.
- Eaves no higher than 3 metres if the extension is within 2 metres of a boundary.
- Materials similar in appearance to the existing house.
- Extensions and outbuildings together must not cover more than half the garden around the original house.
Going deeper: the larger home extension route
You can go up to 6 metres on a semi or terrace, or 8 metres on a detached house, by applying to the council for prior approval first. The council notifies your neighbours and has 42 days to decide. At the time of writing the fee is £249. This route is not available in conservation areas or the Chilterns National Landscape.
Two-storey and side extensions
A two-storey rear extension can be permitted development if it is no more than 3 metres deep and at least 7 metres from the rear boundary, with a roof pitch to match the house. In practice many gardens are too short for this, so most two-storey extensions need planning permission.
A side extension can be permitted development if it is single storey, no more than 4 metres high and no wider than half the width of the original house. It cannot sit forward of the front of the house or a side wall facing a road.
Loft conversions and roof lights
- Added roof volume up to 40 cubic metres for a terraced house, or 50 cubic metres for a semi or detached house, including any previous roof additions.
- Nothing higher than the existing ridge, and no dormer on the roof slope facing the road.
- Dormers set back about 20 centimetres from the eaves where practical.
- Side-facing windows obscure-glazed and fixed shut below 1.7 metres from the floor.
- No balconies or raised platforms.
Roof lights that stick out no more than 15 centimetres from the roof slope are normally permitted development too. For the design options, see our guide to the types of loft conversion.
When permitted development does not apply
- Conservation areas and the Chilterns National Landscape. Rights are cut back: no side extensions, no two-storey rear extensions and no dormers without permission. Parts of Berkhamsted, Tring and Hemel Hempstead Old Town are affected. Our conservation area guide covers this.
- Listed buildings. Listed building consent is needed for works that affect the building's character, inside or out.
- Article 4 directions. Some streets have had specific rights removed by the council.
- Planning conditions. Many newer estates were approved with conditions removing permitted development rights. Check the original planning permission for your house.
- Previous extensions. If the allowance is already used up, you need an application.
Applying for planning permission
If your plans go beyond permitted development, you make a householder planning application to Dacorum Borough Council with drawings of the existing and proposed house. From 1 April 2026 the fee in England is £548. Fees are index-linked and reviewed regularly, so check the Planning Portal for the current figure.
The council's target for a decision is eight weeks from the date the application is validated. During that time neighbours are consulted and a planning officer assesses the design, its effect on neighbours and the street.
Why a lawful development certificate is worth having
If you build under permitted development, you do not have to apply for anything. But a lawful development certificate from the council confirms in writing that the work was allowed. When you come to sell, the buyer's solicitor will very likely ask for proof, and the certificate answers the question in one document. The fee is half the householder application fee.
Planning is only half of it
Whatever the planning route, every extension and loft conversion needs building regulations approval. Planning decides whether you can build it. Building regulations decide how it must be built: structure, fire safety, insulation, drainage and ventilation. In Dacorum, the council's building control service is provided by Hertfordshire Building Control.
How we handle it
On the first visit we check which route applies to your house, whether any rights have been removed, and whether a certificate or an application makes sense. We organise the drawings and submissions as part of the job. See how that fits into the overall programme in how long an extension takes, or read more about our extensions.
Common questions
Can my neighbour stop a permitted development extension?
Under standard permitted development there is no neighbour consultation, so a neighbour cannot block it through planning. With the larger home extension route, neighbours are notified and the council considers any objections about the effect on them. The Party Wall Act is separate and can still apply either way.
How long does a planning application take in Dacorum?
The target for a householder application is eight weeks from the date it is validated. Allow extra time before that for drawings, and a little after for any conditions to be signed off.
Do I need planning permission for a garage conversion?
Usually not, as long as the work is internal and the outside keeps a similar appearance. Some newer estates have planning conditions that remove that right, so it is worth checking the original permission for your house.
Is permitted development the same as building regulations?
No. Permitted development only answers the planning question. Every extension and loft conversion still needs building regulations approval, which covers how it is built rather than whether it is allowed.
General guidance for homeowners in England, written in September 2026. Rules, fees and prices change, and every house is different, so we confirm everything for your property on the free site visit.
