Going back about 30 years ... and I didn't know about it at the time, but ...
At the time I lived in a new development of flats and studio apartments ... they were arranged three floors high, one flat either side of a central stairwell, so six properties in a block. The blocks were arranged so that two blocks (of six flats) were joined. I got to know one of the other residents via a common interest, and he told me about the people in the flat opposite his, which was the next block along from mine. It turns out that it had been bought (or rented) by some cabin-crew from a middle eastern airline, so that they'd have somewhere to live when they were in London. It seems that they'd managed to get their schedules worked out so that when they were away on flights some of their fellow cabin-crew would stay there.
It transpired that some of the cabin-crew were using the place as a knocking-shop while they lived there!
The neighbour (who told me about a few years later) said that he'd hear muted voices through the thin walls.
So, how far was it ... maybe 100 feet?