Log cabins are generally classed as non-residential garden houses. If built as a gym or home office, it won't require planning permission. However, if you plan to live in it, or it'll cover over 50% of your garden, it will need planning permission.
To avoid planning permission, your cabin must be no further forward than the front of your house and occupy less than 50% of your garden. The eaves height can't exceed 2.5m or 3m for the main roof – unless it is dual-pitched, which can be up to 4m.
As a rule, log cabins are not insulated because the thick logs offer enough natural warmth. Each solid wood structure captures air bubbles, keeping the cabin warm in the winter months and cool in the summer.
Your base should be flat, square and level. Concrete bases are ideal because they require little to no maintenance. But paving flags or decking will work just as well.
As a rule, log cabins don't require foundations. But they do need a solid base between 80 and 100mm deep to avoid sinking into the ground. This could be made from concrete or paving slabs. Or be arranged on a patio or decking.