I'd definitely choose a builder with considerable experience in basement dig outs - if not a specialist. 18" may not sound a lot but it very likely takes you below the existing foundations, and any new foundations will have to be quite a lot deeper still. And then the soil pipes often run through the basement. And then there is the waterproofing. Etc...
Make sure you have a competent engineer. And probably plenty of cash.
Friend is considering this in Appach rd. But ruled it out.
As Rushy said, it's costly. Doing it properly might mean fitting a membrane to exclude water, then you need a pump etc etc.
The basement is there already and we have our washing machine and dryer in there, a freezer, a load of shelves etc. I'd say it is about 5'11" in height as it stands. The back of the house is quite a bit lower than the front so you walk at street level in to our kitchen, but it is one storey up at the back. There are a set of brick steps leading from the kitchen, over the basement, to the garden so if we could take these away, we could put direct access from the new bedroom to the garden and there would be natural light etc.
The soil pipe is right in the middle of the basement itself, over toward the far wall, so wouldn't need excavated to be diverted.
The building itself is a 1910s three storey house with a mirror image house attached on the other side. The same area in the mirror image house has been turned in to a room, in fact there are two flats in the building next door where we have one. We live on a hill though, so the basement next door might have been a bit lower down and could have had more head-room and not needed excavation. Still, I reckon there is every chance that place had theirs done.
We've got absolutely no money at all, but intend to remortgage to do the works. The price of the flat has gone up a good bit so we have a good LTV to go to the bank with, but if there are cost overruns then we would be in trouble.
And we need to work out whether it is worth having a four bed flat. One the one hand, another usable room with natural light and access to the garden has to be worth it, but on the other, it's still a flat. Do people buy four bed flats when they could get a three bed house?