How We Calculate Overlay Prevalence

When your property is clear of a planning overlay but many nearby properties aren't, we surface that as Suburb Risk Context — so you know what's common in the area even if your lot is unaffected.

To estimate overlay prevalence, we query Brisbane City Council's public ArcGIS service for each overlay layer across the suburb's bounding area and count the number of features that intersect it. We then divide by the total residential lot count:

When this works well

For overlays where each affected lot corresponds to roughly one map feature — such as heritage listings, character overlays, waterway corridors, acid sulfate zones, and wetlands — this method produces reliable suburb-level estimates. A heritage overlay percentage of 8% genuinely means about 8% of lots in the suburb are heritage-listed.

Where it has limitations

Some overlay layers are composed of many small, overlapping polygons — far more features than there are lots. In these cases, the feature count significantly exceeds the lot count, making the raw percentage unreliable.

What we do about it

We classify each overlay into reliability tiers. Reliable layers have their suburb percentage shown directly. Estimated layers (bushfire) are included with a note that the figure is approximate. Excluded layers (overland flow) are omitted from suburb context entirely — the lot-level check still tells you whether your property is affected.

Your individual property's overlay status is always checked directly against BCC's spatial data and is accurate regardless of the suburb-level estimate.

Data Sources

Overlay data is sourced from Brisbane City Council's publicly available ArcGIS Feature Services. Lot counts come from BCC's DCDB (Digital Cadastral Database) parcel boundaries. Suburb prevalence statistics are precomputed periodically and cached — they are not calculated on every request.

Last updated: see data file timestamp. Method: feature_density. Coverage: 101 Brisbane suburbs.

How We Assess Flood Risk

We query 10 separate BCC flood layers for every property, covering planning overlays, flood awareness classifications, and historical flood events. This gives a comprehensive picture that goes beyond what most property reports provide.

Flood Awareness layers (risk classification)

BCC publishes flood awareness data that classifies every mapped area by likelihood of flooding. These use Annual Exceedance Probability (AEP) thresholds:

What we check

For each property, we query these layers against the actual lot boundary (not just a point):

All flood data is queried in real time against BCC's public ArcGIS services using the property's actual lot polygon boundary. This is more accurate than point-based checks used by most property report services.