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:
prevalence % = overlay_features ÷ total_lots × 100
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.
| Overlay Layer |
Reliability |
Why |
| Heritage | Reliable | One feature per listed property |
| Character (TBC/DHC) | Reliable | Large zone polygons |
| Waterway Corridors | Reliable | Large corridor polygons |
| Wetlands | Reliable | Distinct wetland boundary polygons |
| Acid Sulfate Soils | Reliable | Large soil classification zones |
| Koala Habitat | Reliable | Broad habitat area polygons |
| Biodiversity | Reliable | Area-based ecological zones |
| Landslide | Reliable | Slope-area susceptibility zones |
| Creek/Waterway Flood | Reliable | Large flood planning area polygons |
| Bushfire Hazard | Estimated | Overlapping severity zones inflate count for ~10% of suburbs |
| Overland Flow | Excluded | Thousands of narrow flow-path polygons per suburb — feature count vastly exceeds lot count |
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:
| Classification |
AEP |
What it means |
| High | 5% | 1-in-20 year event. Expect significant insurance loading. |
| Medium | 1% | 1-in-100 year event. The standard benchmark used by banks and insurers. |
| Low | 0.2% | 1-in-500 year event. Rare but possible. Some insurance impact. |
| Very Low | 0.05% | 1-in-2000 year event. Minimal practical impact. |
What we check
For each property, we query these layers against the actual lot boundary (not just a point):
| Layer |
What it tells you |
Tier |
| Overall Flood Risk | Combined risk from creek, river, and storm tide | Free |
| Creek Flood Risk | Creek/waterway-specific flood likelihood | Free |
| Storm Tide | Coastal/river storm surge risk (bayside suburbs) | Free |
| Creek/Waterway FPA | Formal planning area category (1-5) | Free |
| Brisbane River FPA | River-specific planning area category | Free |
| Overland Flow | Stormwater flow path with impact level (High/Medium/Low) | Free |
| 1% AEP Extent | Combined 1-in-100 year flood extent (the insurer benchmark) | Free |
| Flooded Jan 2011 | Was this lot in the 2011 flood extent? | Free |
| Flooded Feb 2022 | Was this lot in the 2022 flood extent? | Free |
| Flood Summary | Plain-English paragraph combining all flood data | Free |
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.