The plot itself is certainly big enough, although you don’t say how much garden your existing house would be left with. The access point, at 2.1m, is below the normal width for a private drive but might be acceptable if there’s nothing on either side of it – such as a wall or gateposts, for example. You’ll need to check this with the council. Look at your title deeds to see where your ownership ends. Then, assuming the cul-de-sac is an adopted road, ask the highway authority to confirm the extent of its ownership. If the two ownerships adjoin, there’s no ransom strip. Check if your plot is within the development boundary of the village by looking at the local plan proposals map, either on the council’s website or at its planning department offices. Finally, the value of the plot is likely to far exceed the reduction in value to your house. To verify this, talk to a local estate agent.