Jelly,
Extensive renovation means u are changing the flooring, electrical works, redoing toilets, kitchen wall tiles etc? Doing wardrobes and all. Then be prepared to blow at least 40k for a 4-room.
My girlfriend upon reaching 35, got herself a 3-room resale and did it all up. She blew $35k! That's excluding furnitures and curtains and electronics like aircon, TV, fridge.