No, you should be fine in most situations. There are certain circumstances in which you might need to seek approval from your local council to build your log cabin, such as if you're planning to live in it. Other than that, as long as your building isn't more than one storey and 2.5m tall at the eaves and 4m tall at the top of the apex (3m if it's a pent building), and it isn't closer than 2m to your house and it isn't in your front garden, you should be okay. Check out this detailed article for more information on building regulations if you're unsure.