In many cases when building a garden room, you can build under the Permitted Development rules rather than having to make a Full Planning Application. The Permitted Development rules state the maximum heights for different shapes of garden room and where you can position them.
By far the most popular option when building under the Permitted Development rules is to build a 2.5m garden room. This is your most flexible option when it comes to positioning your building, as you can site the room 500mm from the boundaries of your garden.
The measurement is taken from the highest piece of ground immediately adjacent to the room; the rule is defined by the Planning Portal as:
Ground level is the surface of the ground immediately adjacent to the building in question. But where ground level is not uniform (eg if the ground is sloping), then the ground level is the highest part of the surface of the ground at the front of the building. Ground level must be virgin ground, ie. not from top of paving or decking.
One of our previous garden room projects was built to the 2.5m rule, a neighbour complained that the building was taller than allowed. The site where the garden room was built sloped from front to back. This slope meant the measurement at the front of the room was 2.5m, but at the rear of the room, the measurement was 3m. The neighbour felt that the 3m measurement convened the Permitted Development rules.
The Planning Officer came out to look at the situation and confirmed that the building was fully compliant with Permitted Development and explained to the neighbour that the maximum 2.5m measurement was taken from the highest piece of ground adjacent to the building, in this case at the front of the building.