How much can I legally increase the rent at renewal?

Only per Decree 43/2013, benchmarked against the RERA/Smart Rental Index. If your current rent is within 10% of the market rate, no increase is allowed; 11-20% below permits 5%; 21-30% permits 10%; 31-40% permits 15%; and more than 40% below permits a maximum of 20%. The official RERA calculator output is legally decisive.