Site Analysis

Flood risk will determine whether and how a site can be built on.

Flood risk occurs in low lying areas of land such as:

• flood plains
• by rivers and watercourses
• below a hill
• close to a valley mouth
• close to drainage channels
• natural depressions or swamps that have been drained.

If a site is prone to flooding, decisions must be made about:

• whether the site is safe to build on
• where to locate a building on the site
• the finished floor level – the council may have specific requirements under section 72 of the Building Act 2004 (section 36 of the 1991 Act)
• landscaping and drainage.

Check council records for information such as the LIM and past flood risk. Consider the impact of flooding on stormwater and sewer systems and erosion. There is provision under section 72 of the Building Act 2004 (section 36 of the 1991 Act) for the council to refuse to grant a building consent if the land is at risk of a natural hazard, such as erosion, flooding, subsidence, or slippage, or if the building work itself is likely to accelerate the problem.

Minimising the risk

If construction of a new building goes ahead on a site prone to flooding, minimise the risk by:

• ensuring the building is located on the highest section of the site
• building away from natural drainage paths or channels
• making the finished floor level of the lowest floor well above (600 mm minimum) the maximum flood level
• installing additional land drainage for low-lying areas (there must be somewhere for water to drain to).

Note: Owners of buildings in flood-prone areas may have difficulty in obtaining insurance.