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What's involved in building a new home in a flood prone area



You may be reading this because you’ve found out that your building site is in a flood prone area, or you may be reading it for initial research so that you’re prepared when you do find land. But regardless of why you’re digging into it, it’s an important consideration, and it’s worth finding out the answer to the question of what’s involved in building a new home in a flood prone area.


Bear in mind that this also covers land that’s subject to inundation – technically there’s a difference from a council perspective, but for the purposes of what we’re talking about here, it’s the same thing – it’s land where there’s a known potential for flood waters to flow across at least part of your property in the event of a serious flood. This ‘serious’ flood is sometimes known as a 1 in 100 year flood, which means that it’s likely to happen only once in 100 years. Of course, land that’s flood prone or subject to inundation doesn’t only cover this scenario – it takes other things into consideration as well.


But let’s start at the start.


Your builder or architect ask the council for information on the property, and one of the items on their response is that the site is flood prone. And – or – you’ve also identified from the government’s land information website that the land has a Land Subject To Inundation Overlay (LSIO) on it. Either way, you’ve now got clear confirmation that it’s definite that at least some of your land could get really really wet at some point in the future.


And it’s important that you get this clarification from the council early on – in fact, you can’t apply for a building permit for your new home without written confirmation from the council that they’ve checked whether it’s in land that’s flood prone. Of course, only a small number of sites are, but every single building permit that’s every issued needs to jump that little hurdle first.


So what to do next? There’s two specific things that you need to look at.


The first is to analyse whether this is going to significantly affect your build.


If the site is really subject to lots of water during a 100 year flood, then it could mean the difference between building and not building, depending on the type of house you want and where you want to put it.


Let’s say you have a couple of acres abutting a creek, and when this creek turns into a boiling torrent every few decades, it spills its banks onto your property and the back part of the property is under water. But maybe you decide you’ll be building on the front part – which is higher – and so all is well. But maybe you want to build where the water comes; you need to then look at a home on a suspended floor, not a slab on the ground. And even then, it may be questionable whether it’s allowable, depending on the depth of the water and the flow rate of the flood waters.


Without spending too much time on it, it’s critical that you get a rough idea of what the implications will be so you can make a yes or no decision. Don’t worry about technicalities yet, but chat to the council and ask whether they can help you with information.


So lets say you decided that it’s all good – either the location of the house is outside the flood line, or even if it is, it’s minor and the suspended floor country style home will provide for this. So the next thing then it to get the relevant approvals so you can actually build the home.


It’s important to be aware that even if your home is outside the flood line, or you have a building envelope on your property denoting where you can build, or even if the council says you don’t need a planning permit due to the flood prone overlay, you still need to get a special approval to build on land that’s subject to inundation or is in a flood prone area.


There’s a few steps involved in getting this approval.  


  1. Get the plans drawn up of the home on the site, showing all the relevant detail of contours, measurements and existing trees.
  2. Send the plans to council and ask for written advice on whether you need a planning permit because of the flood prone overlay. Generally this only applies if the location of the house is actually in the area.
  3. Lodge a planning permit if needed, otherwise jump to the next step. Sounds like a board game, but it’s not quite that easy! Bear in mind that a planning permit may be required for other reasons, which means that everything is assessed in the one application.
  4. If you don’t need a planning permit, then you’ll need to apply for what’s called a Report & Consent. This basically means that you’re reporting to the council and gaining their consent. Send the application form in along with the plans – they’ll ask for money for this application – and then they’ll register it into their system.
  5. The council will then refer it to the water board, who will assess it and provide both advice and details back to the council. There’s really two things the water board do – they look at the potential impact on the house, the occupants, etc and assess whether they will or will not support someone building a home in that spot. And then if they decide they can support it, they specify a minimum height at which the house should be built. This height is called AHD, or Australian Height Datum, which in broad terms is a height above sea level.
  6. When the council gets the information from the water board, they issue a consent to build in land liable to flooding – provided of course that the water board supported it – and this includes the specified height of the floor level of the home.


Great stuff! But even though you have the approval in your hot little hands, there’s still a couple more hurdles to jump.


The first one is to engage a licensed land surveyor to go to site and identify the exact minimum height at which the house must be built. They do this by using their fancy equipment to work out how high above sea level your land is, and then narrow it right down to the measurement given by the water board, and then they put some sort of peg or marker in an obvious spot with this AHD level on it. This means that when the builders come to build the home, they can use their laser levels to reference against this and make sure the floor of the home is at least that high or higher.


In some situations, because of sloping land, especially on larger properties, where you’re building the home may be way above the minimum level specified by the water board, and the house is just built as per normal with no additional considerations. But in other scenarios, especially where the house is going to be subject to flood waters actually running underneath it, then it’s really critical for the house foundations to be set up very carefully.


The other thing that may be needed is some extra work by the structural engineer. If your home is a suspended floor style, and it’s going to get hit by flood waters every hundred years or so – although you hope not! – then the engineer will need to crunch some numbers to work out if the foundations should be stronger to resist the pressure of the rushing flood water. Sounds scary, but it literally means designing for that hour or two nightmare that might happen once every century or two.


Often nothing needs to happen because the home is above the AHD, and even then, it might be in the water area but it’s not subject to high flow rates. But we have seen a couple of cases over the years where some additional strength needs to be given to those footings.


And that’s about it! One thing to understand is that you can’t get around the requirements to have a permit in place; have the surveyor come to site, etc. It’s an important permit area that every single building surveyor is aware of.


As always, reach out for more information!

Disclaimer. This blog is our opinion only. The information provided in our blogs is accurate and true to the best of our knowledge, but there may be omissions, errors or mistakes. The information presented in our blogs is for informational purposes only and we are not professionals, so the content we provide shouldn’t be taken as legal advice. We strongly recommend consulting with a professional before taking any sort of action. We reserve the right to change how we manage our blog and we may change the focus or content at any time.

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