There are a range of projects you can build without planning permission, including many internal works that do not alter the building exterior.

A few examples of common work that you can carry out under permitted development are listed below:

• Single and double-storey extensions that extend only up to 8m from the original rear wall (for detached houses) and up to 6m for all other house types.
• change the function assigned to space. For example, conversion of lofts, garages, basements, etc. 
• interior alterations of space, such as knocking down internal walls.
• The addition of dormer windows 
• The addition of rooflights
• Building a porch with a net area of less than 3 m2
• Erection of antennas or installation of satellite dishes
• Installation of equipment such as solar panels (not including wind turbines)

However, it is always advisable to cross check with your local authority or an experienced architect/surveyor on general guidelines that apply to your property before you begin. If your proposal falls outside the scope of permitted development, you will need to apply for planning permission.