If your property is similar to others in the area this tool below will give you a reasonable indication of value. Add in the value of your extension, perhaps mark it down due to the lack of parking your neighbour has and you have your guide price.

If your property is unusual, isolated, very large or overly complex then consider engaging with a “Buying Agent” or a “Search Agent” in your area. These are the modern alternative to a local estate agent. 

At the end of the day you have to make a decision and going with the agent who gives you the highest value is likely to result in a reduction after they get the listing. 

When properties sell in a few days they may be too cheap or the demand in your area for housing like yours is high ….. Why not List it yourself?