I have an existing landed intermediate single story terrace that is fully paid up. Due to the age and condition of the house (over 60 years, evidence of termite infestation) and the need for more space, I am planning on demolishing the existing structure to rebuild as a double story unit.

Some questions that I have:
1. What would be a reasonable budget for this exercise? The land size is 20" x 70". Surveying around my area, the average pricing I see is between RM200-300k excluding interior design. However, my architect has advised that this may be on the low end due to the need to demolish the existing structure.
2. Termites have been eating through the rubber casing of wires in the house. My initial plan was to demolish and rebuild from scratch, but my architect is advising me not to knock down everything as some of the structural elements can be retained to save cost. Should I be concerned about termites here, or are these elements generally safe from them? I'm assuming this is the case as they're usually concrete with no wood in them.
3. I have been advised by my banker that I can remortgage the property to fund the rebuild. He has however mentioned that banks usually don't give out 90% margin for remortgages - is this true?
4. After discussing with some contacts who have built a house from ground up before, I have been advised to consider splitting the work by stages (foundation, structural, brickwork, etc.) to avoid lock-in with a single contractor and ensure quality of deliverables at each stage. This seems like a lot of work - are there significant risks with going with a single party for the entire build?


Apologies in advance if this isn't the right subforum to post in.