We always suggest consulting your local planners before going ahead with any project.
For garden cabins in most cases, you will not need to obtain planning permission providing the following criteria are met:

• If you're building a log cabin as a separate dwelling you will need planning consent.
• If it is for your own use the following should help:
• Planning permissions depends on the type of use the land is for.
• If you do not have a house on the land then you might want to check with your local council.

If there is a house on the land then following applies:
For garden buildings planning permission is not required provided:

• It is not closer to the road than your house
• It does not cover more than half the area of the garden
• It is not for use in connection with running a business
• It is not more than 4m high
• It is for the sole use of the house occupants
• Your house is not a listed building.

Section B - Buildings and other structures on the land around your house http://www.odpm.gov.uk