Just like birth, death and taxes, renovation surprises are guaranteed. I think Micheal Bell's suggestion to add a contingency is a good one. Also finalising your design before looking for your builder will save on mid project variations. For those interested in estimating the cost of their renovation before going too far into their plans, try this renovation and extension cost calculator. Good luck. richard@makeovergroup (Melbourne)

Regarding the quote. An electrician is a supplier of goods and services, therefore 10% GST is charged on the full invoice amount. The quote shows that the GST of 10% has been charged on total amount, which is correct. So…..why has the quote stated that gst is for parts only? “No gst charged on labour”. This is incorrect and gst is charged on the full invoice, as the calculation on the quote shows. So…does this mean that they have not charged you the labour component? Or, the electrician doing the quote just doesn’t understand the GST system. GST is charged on all goods and services. When the electrician pays their employees this is completely different and has nothing to do with GST.

I would work out how many square metres you are adding and renovating. Then times that by $5000 per square metre. But there are so many variables. What is the house made of, brick or timber, what is the roof line to be able to be adaptable for an extension? Are the floors concrete or wood, to accommodate changes in plumbing? What specifications are you after, low, middle range, or fancy? Is the land flat, do trucks have good access…..?? Etc etc. only a few years ago I would have had said $3000 per square metre, but now…you need good firm quotes….and good firm time lines….so that is why I have said a very unsure $5000 per square metre.

First up talk to your local Council around their requirements. You may find there are hoops to jump through. Have you had a price from a plumber for the sewer connection? That will be the most expensive part. And for materials - there are generations of outdoor loos made out of corrugated iron - so Colorbond would be a solid choice.