I note you give no indication of where the property is located, whether it is really old or new, or somewhere in between.
However, it really depends on whether it is competitively priced for the local market - do you have any idea what similar properties are actually achieving in the local area? If the property has been on the market for a good while, you might expect that it is overpriced for the current market. The other issue to consider is the reports required to be provided by the buyer when a property is placed on the market - is everything OK or are there matters that need rectifying, what is the energy rating etc? If there is work to be done, that would certainly mean that you could negotiate lower (maybe even start at lower than -20% depending on what the additional costs to the buyer are likely to be). If everything is OK, then -20% could be a starting point for negotiations - all you can do is to test the waters.
At the end of the day, though, if you really love the property and really want to buy it, it's going to depend very much on how much you are willing to pay - however you definitely should negotiate. The worst the seller can do is refuse the offer and, if there are no other higher offers you can always come back with a higher offer.