I tried to use several language translation websites and they all gave me different results. Some say "家は綺麗じゃないではありません” means 'the house is not clean', but some say "家は綺麗じゃないではありません” means that 'it's not that the house is not clean.' Which would you say is correct? Any help is deeply appreciated.
The sentence 家は綺麗じゃないではありません is incorrect. You can say 家は綺麗じゃなくはありません (colloquial) or 家は綺麗でなくはありません. Both of them mean the house isn't not clean.
Tho even with the correct structure it doesn't sound very natural (it's not wrong but it seems like an uncommon phrasing). I think most people would say 家汚くはないです