Yes, in most cases you can use an outbuilding as a home office without needing planning permission. As long as the use remains incidental to the main dwelling it is still considered part of your domestic residential use.

Read more about outbuildings

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If you carry out development without the necessary planning permission, and it doesn’t fall within permitted development rights, you may be in breach of the planning rules. In such cases, you will usually need to submit a retrospective planning application to seek approval for the work already completed or started. However, there is no guarantee that permission will be granted.  

Learn more about planning breaches on Planning Portal  

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Deconversion is the process of restoring a previously subdivided house back to its original state as a single property. Merging flats to create a single house may require planning permission, so you should discuss proposals with your local planning authority before starting work.

Read more about converting flats back to a house.

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There is no single legal definition of "habitable room", as its use and meaning are subject to context. Generally, a habitable room is a space in a building that is designed for people to live, sleep, or spend extended periods of time in, but does not include bath or toilet facilities, corridors or utility rooms.

Work that requires building control approval will require a completion notice to be submitted to the building control body. The completion notice must be sent within five days of the work being completed, once submitted, the building control body can visit the site to complete a final inspection.

If the work meets the required standards they will issue a completion certificate.