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.

When installing over top of old ceramic/porcelain be aware that you will most likely have to "sink" the old tiles in cement to do this. That means grinding down the tiles a little (rough up the surface) then the self leveling cement is poured and allowed to dry. The new floor is then laid over top. Vinyl is notorious for requiring EXTREMELY flat surface. A ceramic tile rarely has this requirement (grout lines and "high spots" around tile corners give problems). A glue in place tile is also going to have the same problems and the same solution = sinking the tiles in cement. This adds extra costs to the floor. A rigid floor like a cork floating floor can handle a bit more floor deflection than vinyl or laminate planks. If labour in your area is relatively cheap...you could probably have the tile removed and the sanding of the mortar done for the same price as coating in concrete. You will also have the luxury of being able to lay any floor you want once it is done AND you don't have to worry about increasing the floor height. It is possible to sink the ceramic in cement...but is it the most cost effective option? Maybe not.

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.

We have had designed a 200m2 home and we would like raked ceilings in the dining and lounge areas. If it was a flat ceiling it would be 2155mm high. The lounge and dining are 20m2 each. Roughly, what is the expected extra cost of a raked ceiling v a standard flat one?