(1) There is no zakah on the plot which you bought with the intention of building house. Zakah shall be wajib on the plot which you bought with the intention of selling no problem you intend to spend the amount in building the house provided the condition that make zakah wajib are found.
(2) Whatsoever amount was recived by selling the old house if it reaches up to the nisab i.e. you turned sahib nisab due to that amount. And if you were not sahib nisab before it then from that day one year onward zakah shall be wajib on those amount. In case you bought house from those amount before completing one year or bought agriculture land then its zakah shall not be wajib. In short, zakah becomes wajib after one year from the day on which you became owner of nisab. And if you already were sahib nisab and you received that amount in between the year then it shall be combined with the amount of nisab and the zakah of total amount shall be paid on the date of zakah. In case that amount is spent in some need then its zakah shall be dropped.
Allah knows Best!
Darul Uloom Deoband