Agreement to construct

"I had made an agreement for constr of house in that the there is a clause that this agreement is for six months. the builder has the right to claim 20% extra on the complete construction cost after six months and to stop construction if the payment is not made in time." This clause was taken lightly at the time of signing the agreement due to various promises made verbally by the contractor as also that this is a formality only and we believed his words and signed the agreement. Now even after paying the full amount the constr is held up as the contractor is asking for 20% extra as the period of six month is over now. Now my question is though we delayed the payment, the contractor constructed the house in parts only after receiving the payment. Further the contractor has not given any written notice after six months quoting the clause otherwise we would have paid the amount after six months and would have settled the agreement at that time itself. Now we had started completing the balance work on our own without revoking the agreement and some additional work is also being done by us which was not there in the agreement. What legal action we can take against the contractor as also is the agreement is still valid when the period of six month is over and the contractor has left some work which we are doing on our own. Do help as we have spent all our hard earned money and feel cheated by the contractor.