Hi Neobule

National planning policy guidelines for garages are as follows:

Outbuildings are considered to be permitted development, not needing planning permission, subject to the following limits and conditions:

• No outbuilding on land forward of a wall forming the principal elevation.
• Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
• Maximum height of 2.5 metres in the case of a building, enclosure or container within two metres of a boundary of the curtilage of the dwellinghouse.
• No verandas, balconies or raised platforms.
• No more than half the area of land around the "original house"* would be covered by additions or other buildings.
• In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites the maximum area to be covered by buildings, enclosures, containers and pools more than 20 metres from house to be limited to 10 square metres.
• On designated land* buildings, enclosures, containers and pools at the side of properties will require planning permission.
• Within the curtilage of listed buildings any outbuilding will require planning permission.

If your proposals comply with the above then it is PD. However, the planning policy and application of it is complex and in addition to the above I would look at the planning history on your street, if you fall within a Conservation Area then you need planning. If you are planning on using the extension for more than a garage then I would recommend planning and if the extension forms part of any extension to the rear then I would recommend planning.