As a learner, you should IMO definitely stick to the "canonical" form for remote conditions.
Without more context (and if I were really expressing a condition) my natural tendency would be to say
"It would be nice if the flat were in a nice area.",
though (for social reasons) I would probably be more likely to actually say "was" in real life.
However, particularly with the first example, there might be circumstances where a Brit would say "It would be nice if the flat is in a nice area."
There may even be (reasonable but linguistically complicated) contexts where I would not necessarily classify it as non-standard.
There are a number of complicating issues, one of which is that (pragmatically) " It would be nice if ..." often introduces something other than a proper remote condition; IOW the first example is not really comparable with
"If I found a two-bedroom flat in the Herne Hill area for less than half a million, I would buy it immediately."