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  • Michael Bukowski

How to find out whether my extension is a Permitted Development

Updated: Feb 28


( Atelier 41 Architects' householder guide for a single storey rear extension to existing dwelling house)


In our 15 years experience we very often come across private sector clients that are confused by permitted development regulations ( Permitted development ( England) Order 2015) that regulate development of variety of extensions to an existing house.

A lot of information can be found on the Planning Portal e.g. "Mini Guide to single storey extensions" or in released by Ministry of Housing, Communities and Local Government

" Permitted Development rights for Householders - Technical guidance", but its interpretation may anyway cause a headache to a person not dealing with the development industry on a daily basis.


In a few steps the general and much simplified rules are as follows:

I) Depth of proposed extension:


  • for a detached house - 4 m deep extension measured from the rear wall of the original house


  • for a semi-detached house – 3m deep extension measured from the rear wall of the original house


  • for a terraced house – 3m deep extension measured from the rear wall of the original house


II) Height of proposed extension:


  • 3m overall height for an extension with a flat roof

  • 4m overall height and 3m eaves height( bottom edge of roof) for an extension with pitched roof

In fact, there can be many variations of the roof design that would comply with the above rules. Please see below just a few possible design solutions.It has to be noted they are not exhaustive and another options can be proposed once the client's objectives are established.




The above rules may now seem straight forward but there are many restrictions that cannot be overlooked in order to provide a compliant design.

  • The most important is that most extensions are not permitted forward of the principal elevation or side elevation fronting a highway except for a small porch with external floor area not greater than 3sq.m

  • No more than half the area of land around the “original house” would be covered by additions or other buildings. Sheds and other outbuildings must be included when calculating the 50 per cent limit.

  • Proposed extension ridge and eaves cannot be higher than ridge and eaves of the original house.

  • On many occasions permitted development rights are removed in the planning permission for a new house or in the title deeds for housing built by a housing association ( therefore it is important to get your title deeds checked by a professional like a local architect.)

Moreover, if a rear wall of the original house is not a straight wall e.g has outriggers, single or two storey rear bays, or any other projections or if a developer is unsure whether the rear wall is in fact original or the slope in the rear garden is substantial so that it is difficult to determine at which point should a proposed extension be measured , it is strongly advisable to call the local architect to arrange a site visit. Our skilled team can carry out an initial free of charge site visit and advise all the intricacies in the interpretation of the Permitted Development rules. It has to be noted that every building is different and its relationship with rear projections and unusual features or site conditions may lead to the incorrect judgment and substantial financial loses.

Here at Atelier 41 Architects we deal with such cases on daily basis and even if you have a quick question you are very welcome to give us a call so that we can cast an eye on the proposed development and advise the way forward.

We cover the whole London Borough of Barnet and the adjacent areas and councils like Enfield, Hertfordshire, Harringay, Hatfield and many more. If you are planning your extension there is no better place than our small RIBA chartered architects practice where a dedicated and experience team will answer all your queries and deal with the local council on your behalf.

In more complex cases we also co-operate with leading local planning consultants if their input proves to be necessary.

Our next articles will cover single storey side extensions, two storey rear and side extensions and outbuildings within curtilage of house, so please check our website regularly for further information.


Your Sincerely,

Atelier 41 Architects Team.





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