Postal code service area check

Check whether a postal code falls inside a boundary — a school catchment, a service area, a delivery zone — without asking anyone for their street address.

How it works

Rather than geocoding an address and testing one point, this tests every address carrying the postal code against the boundary. Three outcomes:

Result	Meaning	What to do
All inside	Every address under the code is in the area	Answer with confidence, no address needed
All outside	None are	Answer with confidence
Split	The code straddles the boundary	Ask for the street address, only now

Most postal codes fall entirely within one catchment, so most enquiries resolve on six characters. The ones that split are exactly the ones where guessing would have been wrong.

Why this matters for public-facing forms

A “which school does my child attend” form that asks for a full home address collects personal information it usually does not need. The same form asking for a postal code answers correctly most of the time, and asks for more only when it genuinely cannot.

Less data collected, fewer privacy obligations, and a shorter form.

Doing this offline

SELECT
  count(*)                                          AS addresses,
  count(*) FILTER (WHERE ST_Within(a.geom, s.geom)) AS inside
FROM addresses a
JOIN service_areas s ON s.id = :area_id
WHERE a.postal_code = :postal_code;

Precompute it once and the lookup is a single indexed read. Working with postal codes has the materialized view.