Whenever a fire insurance company shall write a policy covering a building in this State and shall attach thereto the so-called co-insurance clause, or any similar clause requiring the insured to carry insurance in amount equal to any percentage of the value of such building, the insured may ask for a valuation of such building insured, which valuation may be agreed upon in writing by the insuring company and the insured, and shall be the valuation of the property insured for the purpose of fixing the liability of the company during the life of the policy.