How to Check the Overlays on a Block Before You Buy

The costly surprises in residential construction are rarely genuine surprises. Flood levels, bushfire ratings, demolition restrictions, vegetation constraints and sewer alignments are all recorded, and most can be checked before you make an offer.

The reason people get caught is sequence. They buy, then design, then discover. Reversing the first two steps costs very little and occasionally saves the entire project.

Here is what to check and roughly what each one tells you.

Every South East Queensland council publishes property information showing the zone and the overlays applying to an address. It is free, it takes minutes, and it is the single most useful document in this list.

It will tell you whether flood, bushfire, character, heritage, vegetation or coastal hazard overlays apply. Each one carries consequences ranging from a higher floor level to a restriction on demolishing the existing house.

Read it before you make an offer, not during the cooling-off period when you are already emotionally committed.

If flood mapping applies, find out the required minimum habitable floor level. That number determines slab height, driveway grade and sometimes whether a single-storey design is viable.

Do not assume that distance from a river means safety. Overland flow paths — the routes water takes when the drainage system is overwhelmed — affect blocks nowhere near an obvious watercourse.

Flood mapping also affects insurance availability and premium, and how buyers assess the property when you eventually sell.

If a bushfire overlay applies, the site will need a Bushfire Attack Level assessment under AS 3959, and the resulting rating drives requirements for glazing, decks, screens, eaves and cladding.

A high rating adds substantial, unavoidable cost. A preliminary assessment before purchase is inexpensive relative to what that adds, and it gives you room to negotiate or walk.

Bushfire overlays may also impose access and water supply requirements, which matter most on acreage.

If you are buying with a knockdown rebuild in mind, this is the check that decides whether your plan is possible. In Brisbane, pre-1947 houses in character areas and Demolition Control Precincts are protected, and approval to demolish is far from guaranteed.

Where demolition is restricted, the project becomes a raise-and-build-under or a renovation. That is often a good outcome, but it needs to be a decision rather than a discovery.

Heritage listing, where it applies, is a stronger constraint again.

Vegetation protection and, in some areas, koala habitat overlays restrict clearing and can determine where the building sits on the block. In parts of Redland City and elsewhere these constraints materially shape the building envelope.

On a treed block this can decide whether the house you have in mind fits at all, and it affects driveway alignment and machinery access during construction.

Find out where the sewer main runs. If it crosses the block, building over or near it requires approval and specific measures, and relocating a main can end a project. The water authority holds this information.

Confirm there is a legal stormwater discharge point. Where the fall runs the wrong way, the answer may involve an easement over a neighbour's land, and neighbours are not obliged to agree.

A soil test gives you the site classification under AS 2870, which determines the footing system. On reactive clay the difference between a conventional slab and an engineered one is significant.

We run these checks as a matter of course before design, and we are happy to look at a block you are considering before you commit to it. Our Brisbane knockdown rebuild page covers the demolition question in more detail.