The key is to be armed with maximum knowledge.
The internet offers many tools and resources. Some give you the most recent prices that houses in a particular Post Code area sold for such as houseprices.co.uk , mouseprice.com & nethouseprices.com
You can register at the Land Registry and pay a small fee to obtain title deeds for a particular property at landregistry.gov.uk
Other sites give you a bigger picture and list all properties currently for sale and/or sold subject to contract such as Rightmove.co.uk, findaproperty.com; & primelocation.com
I particularly like zoopla.co.uk which gives you all sort of useful statistics and comparisons
But all should be used with caution, they don’t always tell you the whole picture eg if a house was self built then only the price of the land will show or if the house was purchased ‘offplan’ from a Developer and then the house was built under a separate build contract this will distort the figures. Houses sold before renovation or bought at Auction are likely to give a misleading result.
However, helpful all these tools are, they don’t really substitute thorough ‘on the ground’ research and networking with as many people as possible who might give you ‘more knowledge’.
Developers sometimes use a ft2 selling price for their properties and this is often a useful tool – to a point. The larger the house, the less effective the tool.
My own recent experience of establishing the value of a property using most of these tools gave me an end result which varied from £525K to £725k. My gut feeling was that the property was worth £550k
In the end, there was no real substitute for talking to the best Estate Agents who have a more thorough understanding of what types of customer would target the property, what those customers are looking for and what is the most they are likely to pay before they give up and move on to look for something else. Estate Agents also get it wrong sometimes but armed with all of the above knowledge your ‘gut feeling’ will probably serve you well. The Estate Agents valued my targeted property at £575-£600k. So I don’t think I was too wrong with my ‘gut instincts’.