finyo, also be prepared that the answers from them may be much the same if you received here. If I called a local US homebuilder and asked how much it would cost to build a house, I'm quite sure they wouldn't just throw a number out on the table. I wouldn't expect them to. As many here have said repeatedly, there are a number of variables to be taken into account.

Also what type of cladding would anyone suggest?

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.