From the chaotic regions of the Cryptosphere,
(Nigel Jones) wrote on 13 Jul 2003 15:17:30 -0700:

I have submitted plans to build a single garage above what has now
turned out to be a pair of sewers 1m apart
- 300mm surface water
- 150mm waste

It's down to the water company. IMHO, there's a chance that you may
be allowed to build over the 150mm diameter sewer, but that would
almost certainly not be the case with the 300mm. With the both in
combination, it looks even less likely.

If it were able to be built over, the foundations would have to be
excavated to a depth of 150mm below the invert of the sewer. Piling
would be problematic, as they would have to be augered rather than
driven, ruling out a mini-piling rig. If the sewers are very deep (I
can't see the depth being 25", that's only 635mm, which wouldn't be
enough cover on a 300mm pipe), a raft would be a better prospect, as
the total additional load on top of the pipe would be quite low.

One of the water company's concern is gaining access in the future, so
they may well object to a timber garage also (this would have other
problems in terms of Building Regulations in relation to combustible
material less than 1m from your boundary).
--
Hugo Nebula
"You know, I'd rather see this on TV,
Tones it down" - Laurie Anderson