1. If there is no amicable solution, you can send a detailed legal notice to the builder and the flat owner to rectify the problem.
2. Upon receipt of this legal notice, if they do not respond then you will have to approach the court and file a suit for specific performance.
3. Upon the merits of your case, the court will then direct the builder to rectify the leakage problem.
4. If you are ready to accept their offer by bearing 50% of the expenses, then if would be advisable to enter into a written agreement with an arbitration clause, so that if they fail to keep up their promise, you can then seek the directions of the court to enforce the contract.