The eaves in planning terms is the height of the imaginary intersection of the plane of the wall and the roof covering. So imagine laying a piece of wood on top of the roof at the verge, and having another pressed up against the wall. Where they would overlap (on the underside) if they could is the height of the eaves.
Right, so its the underside, but of the highest point of the slope or the lowest, outside face or inside? Over a 4" wall thickness the difference wouldn't be much, just nice to know
I did a very technical drawing
It's the highest part of the ground adjacent to the building. I was reported by a neighbour when i built a workshop at my last house and the only thing they could get me on was the height. I measured 4 mts from 2 courses below damp, where the concrete path was eventually going around the building. The path wasn't yet in, so it was 4.1 (ish) mts to the dirt. They made me lower the roof by 100mm, which i complained about very loudly, but had to do it in the end. Once the concrete path went in, it was 3.9mts from path to top of the ridge board. The roof wasn't tiled when they measured, so it ended up higher anyway, once the tiles and ridges went on. So it's a bit arbitrary really.
So if you keep it 3mts from the highest part of the ground to the boarding on top of the rafters, you will be okay, but then it depends on how fastidious/pernickety the the inspector is if you have a visit.
Oh this part I know for dual pitched, its for single pitch that I'm wondering whether its the front eaves that would be higher than the rear eaves. On a dual pitch both would be the same so all you need to know is whether its to the underside or the top side specifically for the eaves measurement, not the total height.
I'd be unlikely to get a visit at all tbh, I live 100 yards from my nearest neighbor and while a stickler for rules and regs, hes also my father in law! Surrounding me on 3 sides are an empty field and a 4 acre woodland!