Actually, it's not exactly a garden wall. 
I live in a terraced house. On the front of each house is a parking
space (on the property, behind the pavement) The wall I want to build
is between my parking space and the next-door neighbour's parking
space. I want to make the wall as high as legally possible, because he
has an excessively noisy car whose noise I want to deflect as much as
possible. Ok, yes, the wall will be a form of 'subtle hint' but it
should prove quite effective too - and it'll stop him taking shortcuts
over my parking space to reach his own.. he he...

Mike F