Building Height

I am building a garden room away from the main house, the plan is to build to a good standard (current building regulations).

The local planner has told me I can go to 2.5 metres with the flat roof, however when you take out the depth of the floor and ceiling joists, the flat roof run off and felt build up, you are left with a slightly compromised internal height of about 2.05 meters.

There area a couple of things that spring to mind, are there any cheats in the construction, OR can you sink the building slightly by taking the ground level down locally (i.e. where is the 2.5 meters measured from).

This must be within 2m of a boundary then?

If you are quick, and get some footings in tonight, then you can work to the old regulations and can have a 3m roof height!

Otherwise you need to measure the height from the average surrounding ground levels.

It is very unlikely that anyone will notice a slight increase in the height from 2.5m, otherwise you could build the ground up, or lower the building into the ground.

If you allow 150mm for the DPC then 2300mm for the floor to ceiling height, then that leaves 50mm for the roof which is unfeasible. So you need to gain about 150 or 200mm.

Putting the insulation between the roof joists will avoid necessary height

Why did you need planning in the first place? Conservation area or boundary?

Or just being a bit too carefull - sometimes it pays to keep quiet regards planning queries!!

The house is in a conservation area, and the building will be close to the boundary as well. Double whammy.