"These old buildings are usually so solid!" Er...not necessarily. Leaving aside the issue of what are suitable foundations- views have changed over the years - there were probably as many incompetent or crooked builders in days gone by as there are today (and there were fewer building regulations to help them to keep honest)

When I bought a 100-year old house in the 1980's, I had to take down a leaning boundary wall. 4 1/2inch wall with butresses every 10 foot, looked nicely built. When I took it down , the wall was properly founded, but the buttresses didn't go below ground level - in fact you could still see the remains of dried grasses under the bottom course of bricks. Miracle it lasted as long as it did. After that I started looking at "Victorian craftsmanship" with a rather more jaundiced eye. When you're looking for it you can find just as many bodges in an old house as a new one

nick