Outbuildings are a clever way to add extra space and can be added without the need for planning permission, provided they don’t cover more than 50%?of the total curtilage (which doesn’t include the area covered by the main house).
Where you have a larger plot, there may be opportunities to build multiple outbuildings under PD, providing the total area covered by such buildings/enclosures does not exceed 50% of the total area of the curtilage. This 50% should take into account any extensions, but not the area covered by the main house.
Again, outbuildings cannot sit forward of the principal elevation, and there are height restrictions depending on the type of roof (4m for dual pitch roofs, 3m for other roofs, and 2.5m when the building is within 2m of the boundary). Outbuildings may only be single storey, with the maximum eaves height remaining at 2.5m. (See planningportal.gov.uk for more details).
A key factor to bear in mind when considering what you want to achieve from an outbuilding is that the use should be ‘incidental’ to that of the dwelling, e.g. gym, garage, store. Outbuildings under PD cannot be used for residential accommodation, e.g. bedrooms.