What about impervious surfaces, such as extensive concrete drives on some of the higher contour subdivisions, causing additional problems through increased runoff?

Impervious surfaces are an issue that was addressed comprehensively in the recent District Plan Review (2009-2011). The outcome was to address it by using a site coverage rule for the dwelling only (the dwelling can only cover 40% of a property title). Other impervious surfaces such as paved patios and barbeque areas and driveways were excluded because of the practicality of requiring all such projects to get a resource consent(cost and landowner resistance), and secondly the difficulty of monitoring compliance and enforcement. The number of properties likely to substantially cover the whole site is considered to be small and, therefore, not significantly add to the runoff and thus increase the risk of flooding. Notwithstanding, this matter could be reviewed as part of the options going forward.

If a landowner wants to raise their house level now, what level should they use?

What the current information suggests. The modelling allows for a freeboard of 500mm. This is important for looking to the future because of climate change and the frequency and intensity of events.