How much price reduction do you think it would be appropriate to ask for, the agreed price is 285k, the initial asking price was 275k
That's a tough one, and really boils down to the probability of obtaining retention. The minimum would be the total cost to go through the retention process plus an increment for the grief. The upper limit would be the sum of the costs of a failed retention bid, plus the cost of removing the extension, plus the drop in value without the extension, plus overhead for grief.
If it's very likely permission will be granted, veer towards the minimum, otherwise increase towards the max depending on the chances of getting retention.