Whether your annexe is a partitioned section of your home, a completely separate dwelling on the same site, or an extension of your home, local planning authorities are likely to take a number of different approaches when reviewing your case. The positioning of your annexe will largely determine the approach the LPA takes toward your situation.
The Red Tape
The definition of an annexe is that it forms a self-contained yet ancillary part of the main dwelling. It does not matter if it is accessed by an external door or by internal door; an annexe is only officially an annexe if it is joined to the main property. If your granny annex is a bungalow at the bottom of the garden, it is not actually considered an annexe for official purposes!
The key to having a smooth run when approaching the planning process is to be clear right from the outset of your annexe’s existence that you plan to rent it out some day. A recent Telegraph article illustrates how you can run into problems if you end up trying to seek planning permission for change of use from restricted to unrestricted occupation. See the Telegraph for the full article.
The upshot is that LPAs are likely to object to change of use applications, so before you build, extend or purchase a property with an annexe that you intend to let on a commercial basis, you need to have a conversation with the local planning department at your district council. However, if you are using a company that specialises in building annexes like http://www.annexespaces.co.uk/, you can rely on their planning team to expertly negotiate you through the red tape and help you though all the pitfalls of the planning process.
If you are constructing or extending an annexe, you will also have to submit a building controls application. Again, if you use a reputable company, they will do most of this work for you. You will need to prove that your annexe meets all the requirements for a dwelling, such as access, drainage, ventilation and fire safety. As a landlord, you will have legal obligations in all these areas, and if your annexe is attached to the main house, this might require expensive alterations to the house as a whole.