For the roads which are less than 4 meters wide, the plots have to take into account any possible enlargement of the road (until 4 meters, that is). If the road near your property is less than 4 meters wide, a line is drawn on your plot from the middle of the road until 2 meters further. This road is called the “setback”. For architectural purposes you need to consider your plot stopping at the setback. This means that not only you won’t be able to build your property after the setback, but the building ratios (kenpeiritsu and yousekiritsu) have to be computed without the setback area, i.e. with a smaller plot than the one it really is.