Choosing the right comparables radius (and when to break your own rule)

Every radius is a compromise: tighter means more relevant evidence but fewer data points; wider means better statistics from possibly the wrong market. The right answer is a defensible process, not a magic number.

Sensible defaults

• Dense urban: 0.5–1 mile. Markets change street by street; a mile of city can span two different worlds.
• Suburban / market towns: 1–3 miles — usually the sweet spot for scheme appraisal.
• Rural: 3–5+ miles, because the evidence is thin — but index carefully and expect wider ranges.

The boundary test

Before trusting a radius, check what it crosses: school catchments, a railway line that halves commutability, a flood zone, the boundary between a premium village and an ex-industrial town. If the circle straddles a real market boundary, either shrink it or weight by distance so the far side counts less. (Distance-weighting is what Threshold's prediction engine does by default — far comps inform, near comps decide.)

When to break the rule

• Thin new-build evidence: widen the time window (with HPI indexation) before widening the circle.
• Unique product: if you're building riverside apartments in a terraced town, the honest comp set may be similar schemes in comparable towns — acknowledged openly as out-of-area evidence.
• A ground-pricing gradient: where £/sqft visibly slopes across your radius, price your site from its side of the slope, not the circle's average.

Whatever you choose, write it down. "3 miles, distance-weighted, boundary-checked" is an evidence policy; "the comps I liked" is not.

Data referenced in Threshold is displayed under the Open Government Licence (HM Land Registry Price Paid Data, EPC Register, ONS, UK HPI). All figures produced by Threshold are statistical estimates for site appraisal, not RICS valuations.