If the defendant, so claiming, alleges and proves that he and those under whom he claims have had the premises in actual possession for more than 20 years prior to the commencement of the action, the jury may find that fact. In estimating the value of the premises, if no buildings had been erected or improvements made thereon, they shall find and state in their verdict what was the value of the premises when the defendant or those under whom he claims first entered thereon. The sum so found shall be deemed the estimated value of the premises. In estimating the increased value by reason of the buildings and improvements, the jury shall find and state in their verdict the value of the premises at the time of the trial, above their value when the defendant or those under whom he claims first entered thereon. The sum so found and stated shall be taken for the buildings and improvements.