In Barnett v Secretary of State for Communities and Local Government the Court of Appeal had to consider whether the curtilage of a house had been extended by a plan submitted with an application for alterations to the house.

Permission was obtained in 1995 for a new dwelling, and the application included a red line plan showing the extent of the curtilage. In 1998 further permission was granted for dormer windows and a garage/games room. One of the plans accompanying that application showed an increased curtilage.

Subsequently, a swimming pool and tennis court were constructed outside the site of the original curtilage and the local planning authority issued an enforcement notice alleging a change of use from agricultural land to residential.

An earlier case of R v Ashford Borough Council ex p Shepway District Council ruled that if a planning permission was clear, unambiguous and valid on its face, there would be no need to refer back to the planning application or any other extraneous evidence to construe it. However if the permission contained words such as "in accordance with the application and accompanying plans..." they could be taken into account when construing the permission.

The claimant argued that it was mandatory to submit a site plan with an application and all the land shown on that plan received the benefit of the planning permission. Therefore, argued the claimant, the 1998 permission authorised the increased curtilage, as shown on the plan accompanying that application.

The court dismissed the appeal. It distinguished Ashford on the basis that that case concerned an application for an outline permission. The principles established in Ashford were not intended to apply to an interpretation of a full permission.

The proper interpretation of a planning permission was a matter of law involving the construction of the permission and any other document to which it was appropriate to have regard.

The court drew a distinction between permission for a new dwelling and a permission to extend an existing dwelling. The first includes implied permission to change the use if the previous use was not for residential purposes. The second may or may not do so depending on the plans accompanying the application. In Barnett, the 1998 permission related to development shown to be within the curtilage described on the plans with the original permission. As the swimming pool and tennis court were constructed outside that original curtilage, the claimant could not argue that the curtilage was extended by the 1998 permission.