Whether you need planning permission would depend on the size of the log cabin you would like and your local planning authority.
There are national planning regulations but a local authority can vary them depending on the area you are in.
The best thing to do is check whatever you propose to do with the planning department of your local council.
If you are advised that you either require planning permission or they feel you would not get planning permission if required it ask for their suggestions.
Often a small variation in what you propose to do can make the difference between getting what you would like or not.,
I would ring them first ask them what proportions are allowed and if you can check whatever you wish to do with them. Take a note of who you speak to and contact details
This will get the dialogue going between you.