I've touched on this before but i was finding it hard to explain so i've got some crap drawings to help
My house is on a hill with the house ground level higher than the rear garden. In picture one (not to scale!) you can see the garden cut in 2 levels with a car port at the rear with plastic roof. I propose to build a garage with a lower roof the same height as ground level. This garage would be within the 50% of original outside space set out in the permitted development.
There are two problems i see with this proposal. The rear wall height, and the ability to walk on the roof. Because of the slope of the land the rear wall will be only 150-200cm from ground level of the house but the height at the rear lane will be more like 400cm. From where i've read ground level is defined as the level at which the house sits. so this shouldn't be a problem right?
The second issue is the roof being able to be walked on in effect becoming the garden space. Everything i can find regarding walking roofs say you need planning permission because of neighbours privacy however my roof will be at ground level. This wouldn't really apply to me, so would i just need building regs?