Zakah is not wajib on the value of both the houses; however zakah shall be wajib on the rent provided the conditions which make zakah wajib are available with you. Zakah also shall be wajib on both the plots (other than the house) which you bought with the intention of selling and till now your intention is the same. The current market value shall be considered for calculation of zakah. 2.5% zakah should be paid as per the current market rate.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband