Objective Reasonableness: When is an outbuilding reasonably required?

Judging whether an outbuilding ought to be considered ‘incidental to the enjoyment of the dwelling’ is not always easy.

The provision within the curtilage of a dwellinghouse of any building or enclosure, swimming or other pool required for a purpose incidental to the enjoyment of the dwellinghouse as such is permitted by Article 3, Schedule 2, Part 1, Class E of the Town and Country Planning (General Permitted Development) (England) Order 2015 (GPDO).


A common area of dispute is whether the proposed outbuilding is required for a purpose incidental to the enjoyment of the dwellinghouse as such. Consideration of this issue raises two questions: is the purpose of the proposed outbuilding incidental to the enjoyment of the dwellinghouse as such and, if so, is the proposed outbuilding reasonably required for that purpose.


In terms of whether the purpose of the proposed outbuilding is incidental to the enjoyment of the dwellinghouse as such, case law confirms that the keynote is reasonableness. Case law establishes that what is abnormal is not necessarily unreasonable, but also that what could be regarded as incidental does not depend on the unrestrained whim of the occupier. This approach follows that taken in Emin v SSE Mid Sussex, and subsequently reiterated in Croydon LBC v Gladden, in which the Court held that the concept of what was incidental to the enjoyment of the dwellinghouse as such involved an element of objective reasonableness and was not a matter solely at the whim of the individual owner or occupier.


It is settled case law that the size of the building is not, in itself, determinative of whether a development falls within the provision of Class E3. Nevertheless, it is necessary to look at the function of the space within the proposed outbuilding.


In 3158554, the Inspector found that a proposed pool “occupies only some 18% of the total floor area and occupies less space than the changing room, play area and sauna combined. There is also a considerable amount of circulation space that does not surround the pool itself and for which no specific purpose is identified. It follows that a significant proportion of the total floor in the proposed outbuilding, including the play area and much of the circulation space, would not be used for purposes directly related to the swimming pool.”


The Inspector held that:


“The appellant has provided no evidence to demonstrate that the size of the building, in terms of floor area, is reasonably required to fulfil the intended purpose of a swimming pool. The appellant has therefore failed to discharge the burden of proof that falls upon him.”

The Inspector ultimately decided that the proposed outbuilding is of a size that cannot be considered as being reasonably required for a purpose incidental to the enjoyment of this particular dwellinghouse.


Whilst the permitted development right does not require prior approval from the local planning authority, the burden of proof will be on the occupier to show, if required and on the balance of probability, that a proposed outbuilding is reasonably required for its intended purpose. As such, where there is any doubt surrounding your ability to demonstrate reasonableness, it may be prudent to apply for a Lawful Development Certificate before undertaking any work.