On Tue, 13 Apr 2004 11:54:53 +0100, a particular chimpanzee named
Derek randomly hit the keyboard and produced:

I am planning to build a single story garage parallel and as close as
possible to a 2.1m party garden wall. I would expect the garage to run to
15.2m parallel to this wall and parallel to my house. The reason I want to
get as close as possible is that there is only 3.4m between my house and
the wall.

Sorry, I can't visualise imperial measurements (feet & yards at least)

To this end, I will need to dig a trench about 0.9m deep x 0.9m wide (we're
right onsub-soil) right next to this wall which also has about a 0.9m deep
foundation (slightly less). I am of course worried about colapsing the
wall. I get on well with my neighbour and would like to keep it that way.

Provided your foundations are no lower than the neighbouring wall,
then it shouldn't collapse provided there isn't a long period between
excavation and backfilling (i.e., more than a few months). While the
wall is 'exposed', it is acting as a retaining wall, but if it's
225-300mm thick coursed masonry, it will be adequate to cope with most
normal loads anyway. If it's thinner (only half a brick thick) then
it should be OK provided it's not overloaded (i.e., if your neighbour
parks his car next to it, or if you have a few weeks of heavy rain
before you backfill).

Why do you need to be 0.9m wide? A normal strip footing for a small
garage is between 450mm-600mm wide on most sub-soils. If you need to
excavate below the adjoining foundations, then do it in short sections
akin to underpinning.
--
Hugo Nebula
"The fact that no-one on the internet wants a piece of this
shows you just how far you've strayed from the pack".