10% is actually the rule of thumb number that I have heard used for years. Of that 10%, about 60% typically goes to construction docs and the other 40 goes to the earlier phases of the job. Pre-design, if any, is often done hourly. None of this is set in stone anywhere and it will vary depending on the designer's experience and overhead, the complexity and size of the job, etc. There is certainly nothing alarming or unreasonable about asking 10%, and in design, as in most things, you generally get what you pay for.

Well balancing out material and costs labour costs is one thing but 2 story is quite a bit cheaper to build ; far less foundation and roofing area. It also simplifies the plumbing and of course is far more energy effective as downstairs heating will also contribute to upstairs heating. PROs can develop on this. And as Curt points out, modular is a very fast and cost effective solution which can allow you to put your pennies into better quality. Be very careful to select a known and competent professional if you go this way though, i don't know about NZ but europe abounds with cowboys ; low costers are to be strictly avoided.

I'm glad you guys are growing.

We asked a similar question when we bought our piece of bare land. We were advised that building a simple shape would reduce costs... so we've built a t shape. Single story is almost always cheaper. We have a builder who is happy to let us do a lot of work with him. We also choose to live on site, which saves us a lot in fuel. The kids and I clean up the building site most days. We've been able to separate the rubbish out between actual rubbish and recyclable material. As for cladding, we were going to do a mix of corrugated iron with plywood board and batten. But given some of the additional hassles, I don't think there's much in it cost wise. However, in saying all that, we chose to invest in long term benefits, so our house is exactly what we want and should be cheap to run. We were hoping to build for $1800 /sq m but we're still coming out at about the $2000 mark.