If any person, firm or corporation builds upon any land included within said proposed street lines, or repairs or otherwise improves that part of any existing building within said lines, without giving the city council an opportunity to acquire said property free from improvements, as provided in section 10.4 of this article, the council shall not be required to pay for the value of said building, repairs, or improvements in any proceeding subsequently brought to acquire the land for the purpose shown on the officially adopted map or maps.