There is a house on my street that must have had owners for many, many years. About a year ago I saw a for sale sign on it. I always thought it was a nice house from the outside. I saw inside pictures, and it was run down. So, months later the for sale sign was down. The next thing I saw was another house being built in close proximity of the old house. All of a sudden it clicked in my brain that whoever bought the old house, chopped off a parcel large enough to build a house on. Land is expensive and this was a way he bought an old house at a reasonable price, chopped off a lot and built a new home on it. Basically, he got the lot for free! Then sold the new house and probably sold the old house 'as is' and didn't put any money into it. I think this was brilliant! There is a possibility too that there could be town water and sewer too. Where I live there is neither and we had to drill a well and install a septic system. Those are costly too and if the builder avoided that it would have been a cost saver too.
Of course, the builder had to do his homework and find out if it was possible to chop off a portion of the land to build another house. I am sure that cost a bit of money with attorneys to draw up paperwork and dealing with the town.
The builder may put some money into the old house and then flip it for more than he bought it for too.
Can't say I was in love with the extra house in that spot but once it was built and put on the market it sold fast!
I think this builder was very smart!