Do we pay zakah on the total amount saved each year, or just the amount saved that year?

We are a family of 1 sister and 1 brother, my father died early when we were young. We live in a small old single bedroom home provided to us by our grandfather which is now not sufficient for all of us. From past few years we have been saving some money to buy or construct a home. My question is that do we have to pay zakah every year on the saved money at the end of that year or do we have to pay zakah on the total amount which is with us in that year which includes the savings from the previous year too. For e.g. in 2011 we had 2 lakhs and in 2012 we saved 2 lakhs. Will we have to pay zakah of 4 lakhs or only 2 lakhs as we had already paid zakah for the amount saved in 2011?