How to calculate a fair offer?

I’m looking at a house that’s been on the market since the start of the year and hasn’t had a single price reduction. It’s not shifting because it has a few issues that seem to have put other buyers off (very small garden, currently rented out, layout could definitely be improved), but they’re things I could work around.

The problem is there’s nothing comparable that’s sold in that area recently, so I’ve got no real benchmark for a realistic price. It was also a self-build and has never changed hands, so there’s no sale history to go on.

My thinking is that if it had gone through the usual cycle of reductions over the year, the asking price would probably be quite a bit lower by now. I’m planning to base my offer on where I think it should be priced at this point. But because they’ve never reduced it, my offer feels like it’s going to come across as very low.

Has anyone else been in this position? How did you judge a fair offer when the property had been sitting for ages but the sellers refused to budge on price?