It depends on the house and whether the bulk of the work is done by the owners. If they are 'handy' or have access to trades within their social circle, then this is a common occurrence in my area.
They get together, they buy what is affectionately known as a probate house that hasn’t been touched in the thick end of 30 or 40 years, and rip it all out, re plaster, re wire, re plumb, check the roof, add in a low end kitchen and bathroom, minimum tile, and cheaply carpet. This is systematically done over evenings and weekends, usually starting around Easter time (you can spot all the skips being delivered - you just look up your own roads) and takes around 4 to 6 months.
Voila, for very little outlay there is a lot of profit to be made as the labour is nearly always free, the tools are already owned, and the material bought at trade price and off set against personal (self employed) tax liabilities.
If the house is to keep - then they all work on each other houses, in a you-scratch-my-back sort of way, just paying material costs, never labour costs. The sparkie does the plumbers wiring, the plumber does the plasterers boiler, the plaster does the joiners brickwork and so forth, all swapping trades for mutual benefit.
And this is why I never comprehend this forum in its utter pettiness about charging family and friend’s petrol money, or charging for babysitting, or Christmas dinner