3 Tips to Help Determine the Maximum Permissible Area of an Extension in Central London / by Taylan Tahir

Extensions to existing houses can generally be accommodated at the rear, provided they are designed to respect the character and size of the original house.

It is important to distinguish between:

  • Extensions that fall under Permitted Development (do not require planning approval).

  • Extensions that require detailed planning permission.

This post is concerned with defining the maximum area of an extension so that it is likely that the resulting mass will fall under the latter category, requiring planning permission.

Two storey rear extension designed by us at Tatham Place, Westminster.

Two storey rear extension designed by us at Tatham Place, Westminster.

Tip No.1 – Understand The 45 degree rule

The 45-degree rule will be used by the planners as a guide in assessing the acceptability of applications for rear extensions to prevent undue loss of daylight to neighbouring properties, to avoid excessive overshadowing of gardens and preserve a reasonable standard of outlook.

The 45-degree rule is assessed on both plan and elevation. An extension should not exceed a line taken at 45 degrees from the centre of the nearest ground floor window of a habitable room in an adjoining property.

Allowable massing/height of extension

Allowable massing/height of extension

Allowable footprint of extension

Allowable footprint of extension

In practice, and from our experience, the above rule tends to disqualify two storey rear extensions from mid terrace properties. End of terrace, semi detached and detached properties are more likely to be able to accommodate a two storey extension


Tip No.2 – Acceptable Depth

With terraced houses, including end of terrace, the maximum depth, normally acceptable, is 3 metres, provided a minimum of 50% of the back garden remains.

For semi-detached houses the maximum depth normally acceptable is 3.5 metres, provided a minimum of 50% of the back garden remains.

For detached houses this dimension grows to 4 metres.


Tip No.3 – Acceptable Height

The height of single storey rear extensions will be assessed on a case-by-case basis (and subject to the 45-degree rule). However, from our experience, 3 metres height is generally acceptable. This will allow for at least 2.5 metres internal height (floor to ceiling) and a comfortable roof buildup.



The above 3 tips, if followed, will generally yield a massing that is acceptable to the planners. This is, however, a simplification of the issues and there will be circumstances where other issues affect the outcome, such as orientation and sun path, differing heights between neighbours, unique site constraints etc…

Equally, unique site constraints and circumstances can, where combined with clever design, yield greater massing.

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