Several times I've been to take an outside photo of an address where an ancestor lived and noticed a "For Sale" sign outside (especially if the original property has been converted into separate flats). Now I always try and find an estate agent near the property I'm interested in, and pretend I'm an interested buyer if the property or a similar one in the same block is for sale or for rent with them.
I now have quite detailed information/ photos and plans of the interior on one flat in Highbury, one in Hampstead, another in Fulham, and yet another in Eastbourne where my family lived, sometimes over 100 years ago.
And of course the internet throws up all kinds of other opportunities with Estate agents advertising their properties there.
Keith