If you are planning a small scale project, it's always worth considering whether it comes under permitted development rights. The process of gaining planning permission is never a sure thing, so if you can avoid it all together without compromising your design, things can move much more quickly and easily.
There is a great deal of quality guidance out there for permitted development rights - here is a simple breakdown to help you find what you need.
Before you start, it's worth checking that your home is not situated in a conservation area as the rights can be restricted. Heritage sites are also affected, and more information about all these areas can be found on the planning portal. To further complicate matters, local councils have the right to tweak development rights as well which they often list on their websites (in Maidenhead the restrictions can be found here).
In terms of what work comes under permitted development, the ultimate guide is the technical guidance produced by the Department for Communities and Local Government. However, this is quite dry and incredibly exhaustive, so you may prefer the 'Do I Need Permission' section of the planning portal. This has interactive examples and mini guides for common projects which can be an invaluable tool for understanding what is permitted.
Some councils offer an advisory service to check and comment on whether your design comes under permitted development, so if you're not sure then it's a great route to take. This service costs around £60, and while you may have to wait, it can be well worth it (Wycombe and Marlow application page can be found here).
While there is guidance out there to help homeowners, having an architect on side during the early stages of your project will help you navigate this process, as well as pursuing the right course to meet your design brief. Good luck!