It wouldn't be a trespass because you'd be buying the property with the right reserved for those gutters, overhangs, drainpipes etc. to be in place. Builders usually aren't stupid with the legalities and many houses are built this way.
If it was a band new property, it wouldn't happen as it wouldn't pass building regulations.
Unlikely Building Control would even get involved, most garages are exempt from The Building Regulations (if detached, made of brick and less than 30m2 etc). Even if not exempt, the only consideration they'd look at where astride the boundary (in terms of impact on next door) is in relation to the potential for spread of fire, but if the garage side wall is all brick with no windows that'd be a tick in the box. Overhanging soffits would be a civil matter for neighbours to agree on the arrangements.
Ok maybe a better question would be what is the legal standing assuming both houses are still for sale?
As I didn't want to leave you guys hanging (see what I did there), the answer is in the Party Wall Act Section 2(2), which gives the owner the right to:
(h)to cut away or demolish parts of any wall or building of an adjoining owner overhanging the land of the building owner or overhanging a party wall, to the extent that it is necessary to cut away or demolish the parts to enable a vertical wall to be erected or raised against the wall or building of the adjoining owner;
In that case, you would simply have to serve notice on the neighbour. As it's your right to do this, they can't actually object. The only proviso is that you then have to make good the part of their property that you cut, which is to be expected.
Satisfactory solution, if not very irritating.
(added this to the OP in case it's of use to anyone)
This is awesome to read mate
Were lookgin at an extension and one of our big concerns was the neighbours soffits and gutters on our side of party wall!
Ive saved this for future use so thanks for reporting back