Hi,
Yes these are fairly easy, cost will start at around $200k to whatever you want to spend depending on the finish you are after.
Can't comment on will it add value to your house as I don't know the QLD market, in Sydney Yes, we have done a lot and every one has added more value than it cost to do it.
Check the real estate for your local area, compare what you want , ie 4 bed 2 bath etc and see what equivalent houses are going for, work out a price then you will know if it adds value.
If you intend on living there for many years it doesn't matter if it adds value straight away as it should in the future.