UK Planning states that an outbuilding must remain below 2.5m in height, otherwise planning permission is required.
My problem is that the ground is uneven, but the roof would be flat, so where do I measure this height? Should it be an average of the tall side and short side, or should it be the tallest side alone?
Best Answer
Edit to answer new question:
It appears that currently you'd measure the height from "the highest part of the adjoining land.", which would mean that you'd measure from the up side of the slope. Source.
So your building; as planned, is in compliance.
Original answer:
The verbiage of the laws is very important. After some research, I think I've found what you're referring to.
Which means for a flat roof shed, you can have an overall height of 3m, with an eaves height of 2.5m.
Furthermore:
Quoted text comes from the Outbuildings section, of the Planning Portal.
NOTE: Check with the appropriate government agencies before starting this, or any other project.