We had a small two bed flat in London, four floors up and with no lift, only the stairs. We were owner occupiers and it was in negative equity at the time.
We rented it out and moved when DD1 was a year old. With help from my parents we managed to buy a three bed house out of London, which was far more suitable. We continued renting the London flat out for seven years, by which time it was well out of negative equity and in good profit. The sitting tenant bought it off us and we were enabled to buy the four bedroom house we currently still live in.
All three of our DDs have grown up here, though two have virtually left home now and the other might be considering it. We'll downsize in the next few years, I think.