It shall be unlaw­ful to reduce or dimin­ish the area of a lot or plot of which a plot lay­out has been filed and has been used as the basis for a per­mit, unless a revised plot lay­out show­ing the pro­posed changed in con­di­tions shall have been filed and approved; pro­vid­ed that this shall not apply when the lot is reduced by rea­son of a street open­ing or widen­ing or oth­er pub­lic improve­ment.