There are a couple of structural implications of building on small allotments (under 450m²) that your engineer needs to consider. We’ll help you identify some issues with small allotments so that your build remains trouble-free.
The minimum allotment size in Brisbane has been reduced in the Brisbane City Council – Brisbane City Plan 2014 (City Plan).
Let’s go through some of the issues that might arise on a small allotment.
Undermining Footings
There are a few suburbs in Brisbane that are flat.
But if your new small allotment is sloping your engineered footing design needs to take into account not just the footings of your building, but those of your neighbour.
The side of this house in Denver collapsed when the building contractor inadvertently undermined the footings of the neighbouring house.
And then there is this disaster in Mount Waverley.
Prevent something like this from happening to you:
- Ensure your engineer considers the ‘big picture’ when designing your footings.
- Are they aware of the proximity of neighbouring properties?
- Have they ‘poked their head over the fence’?
- Have they considered the location of sewer and stormwater mains?
- Have they assessed the ground level and floor level in relation to neighbouring buildings?
- Have they documented a method for stabilisation of existing adjacent structures?
- Don’t blindly trust an expert. Make sure the information you are getting is sensible and specific to your situation.
- Make sure your building contractor is aware of the proximity of neighbouring properties.
- If you become aware of a potential problem, call a “Stop Work”. Kick the builder off the site if you have to. The owners in the video had a better overview of the potential problem. Taking a photo sometimes just isn’t enough.
- Get a qualified structural engineer on-site to assess any potential instability issues.
- Engage a building consultant or an engineer to undertake a condition survey for neighbouring properties before work starts. This survey protects everyone!
Retaining Walls
Retaining walls need to be designed for your specific location.
I’m sorry, but a generic design for a retaining wall won’t suffice when a building is located in close proximity to a retaining wall. The weight of the building needs to be factored into the retaining wall design.
Once again, the excavation for the retaining wall needs to consider whether the neighbouring property will remain stable during and after the construction of the wall.
Check out this article in HiPages that has information on building a retaining wall on a boundary.
Reconsider your need for a flat building pad on a sloping site.
Perhaps waffle slabs aren’t the best choice for a sloping allotment. Ask your builder to consider the alternatives or call us for a chat.
Swimming Pools
Carefully consider whether the excavation for the pool on your small allotment will affect the footings of your house. An engineer can help.
Next; Carefully consider whether the excavation for your pool will affect your neighbour’s property.
Just recently we were sent a proposed site plan for a pool. Did the pool affect the pool owner’s house?
No.
It was positioned near the fence and far enough away from the house.
Unfortunately, the house next door was built right against this same boundary! A pool right next to the fence could have undermined the neighbour’s house, collapsed the pool wall or both!
Either stick your head over the fence and have a look at the neighbour’s property or use Google Earth to check the location of adjacent properties.
Building Instability
A tall, skinny building on a small allotment is a lot less stable than a sprawling mansion.
The City Plan allows buildings up to 9.5m (check your own site requirements – don’t take my word for it!).
With narrow allotments, setbacks and clients attempting to catch some backyard views, the potential for building instability becomes a real issue. Structural design of these buildings really should be done by a structural engineer.
In any case, I beg you: Do not eliminate all of your bracing walls and replace them with windows.
If your building designer or architect thinks the view from your lounge room is more important than the stability of your building, consider the resale value of your home if it collapses.
Bracing walls need to be spaced throughout a house in both directions. They become super-critical in 9.5m high buildings. The ground floor has about 2.5 more bracing load on it than the upper floors.
Use a structural engineer to calculate your bracing loads and layout bracing walls and/or a bracing system.
Just remember, bracing walls are much cheaper than cantilevered portal frames or braced ceiling diaphragms.
Underground Infrastructure
Re-purposed residential allotments traditionally have underground infrastructure running through an allotment.
The unused space in a backyard used to be the ideal place to run sewer mains.
That space is now getting built out and building over sewers is becoming more common.
Ensure your engineer does a Dial Before You Dig on your allotment.
It’s free and easy.
We already check every renovation and new house project for underground services.
Small Allotments
Building on a small allotment has some challenges – but these can be overcome by careful design and thoughtful building contractors.
If you have any concerns with the design or construction of your property on a small allotment, call an urgent “Stop Work” meeting before something collapses.
If you need some independent advice regarding your small allotment construction in Queensland, call us for a chat.