You can ask for whatever you like. Whether you’ll GET what you ask for – that’s a different question.
It’s part of our system that a buyer can pull out at virtually any stage. While that enables frivolous offering at times, it also covers for example the situation you have above.
Just suppose what you are asking for was the norm. The biggest purchase of someone’s life is then based on what – a 20 minute look around? If they then get an expert opinion, (at their cost, btw) who says it’s in need of millions of pounds worth of work to even make it not fall down in the next year….. you’re then saying if they pull out on that basis you want your fees covered, or if they don’t pull out then they can’t renegotiate based on the new situation.
That to me seems more unfair, if i’m honest, and I’d tell you to get stretched at that point.
IMHO you can start to consider someone is serious when they pay for a survey – which is money they don’t get back if it turns out you’ve ****ed the house up by cutting out supporting girders or whatever. Up until a survey is done – price isn’t agreed, and the house can remain available in my book.