Some of the detached houses they are building today are dire.
The space between them is appalling, the gardens are tiny and in a lot of places, will not sell.
There are many young families in need of affordable three or four bed homes, or may just need a bit more space and this is the crux of the extension economy.
I would sooner live in an extended three bed semi than live in a detached dump that i have had to pay another 40 grand on top of my extension for the privilege. My uncles new house is less than 900mm from his neighbour and the back gardens are tiny.
The houses they are building now are a disgrace and are the slums of the future.
However, i do agree with Salad spoons that you can not polish a turd. Adding a 25k extension to a house worth 50k is not financial wizardry at its best. But people do do it and that is because they are happy where they live and want to stay put.
P.S. i build extensions for a living and get a lot of feedback. The main point being that people have found a good home in a nice area and just want a bit more space. This accounts for a large percentage of the extensions i build.
Also to add, one or two have moved since building an extension i have built though i have not had the chance to ask if the extension helped sell their house or if it added a great deal to the overall value.