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Possible extension - Draughstsman

We have a small 3 bdrm home on the North Shore. As our young boys are growing, we need a little extra space, so are thinking about possibly of adding on an extension with a master bdrm and ensuite, and possibly connecting this to the external garage which could then be converted to a second living space. I'd like an idea of cost for a Draughtsman to assist with plans/permits etc. We have been thinking about this for a while but dont really know where to start. At this stage we would like some help/advice: a. Advice on the process b. Advice on how best to do this - we have an idea of what we would like to do but not sure if it is possible or not - c. possibly someone to help draft plans etc d. Idea of cost/timeframes to do so At the moment, the biggest concern is cost - we would need to be sure this is a more cost effective option for us than buying a larger home.