We are looking to make an offer on a house (the millionth offer we've made in this awful year after our previous purchase fell through due to COVID last year! 
). It's a bit of a quirky old cottage that has a new ish extension on one end. The ceilings in the new bit are nice and tall, but there is a considerable difference to the old bit, where the ceilings are quite low. We are considering whether it would be possible to dig down and lower the floors in 3 rooms.
Has anyone done this? How much did it cost/how many square metres? The floor is various wonky heights anyway, so would make sense to even it out, but if we could add some headroom, that would be great. The house is unlisted and detached, there are no near neighbours, so I would think relatively straightforward from a planning perspective.