Implications of buying house/land on an area where there is a road planning

I've been looking at buying land and building a house in central Tokyo for about a year now. We have some specific requirements like big roof or deck balcony and a garage for big cars. So it turns out to be a pain if you want to build in a nice area and you don't have an unlimited budget. We just came across a second hand house that ticks all the boxes. The problem is that it's in front of an 8m road and the city is planning to widen the road to 10-13m. According to the agent the plan was initiated in 1980 and nothing has happened yet. We called the city road planning department and they couldn't give us any helpful information exactly because of this. Nothing has changed since the initial plan. Since I will be repaying this for the rest of my life and there are too many unknowns I'm thinking of walking away. But still I want to consider a few things if anyone knows what happens in these cases.

– Does the city enforce a setback in your land and compensates you at market rate? What about if the remaining land gets too small and becomes unattractive to buyers? Do they compensate you for this as well?

– Or does the city buy your entire land/house at market rate?

Anything else I am missing or I should try to find out? As I said I'll probably walk away but the house is perfect and it's probably about 5%-10% below the prices in the area because of this planning I guess. So I want to make sure I am not overthinking the risk factors here before I make my decision.

by muku_