Where in Harrow does a house extension actually get determined?
Harrow gets more larger rear extensions notified to it than any London borough except Redbridge — 1,194 determined in the twelve quarters to March 2026 (MHCLG planning statistics, retrieved 1 September 2026). That is what a borough of interwar semis on deep plots produces. The table below sets out each postcode district, the stock that fills it and the authority that determines an application from that address. The fees quoted are the statutory England rates for 2026, not a price for the extension.
| Postcode | Area | Dominant stock | What it means for a house extension |
|---|
| HA1 | Harrow / Harrow on the Hill | Georgian and Victorian houses, listed buildings | HA1 takes in Harrow town centre and Harrow on the Hill. Both are borough addresses, and the council determines them against its own Local Plan. |
| HA2 | North Harrow / South Harrow | 1930s semi-detached | North and South Harrow both sit inside HA2, and HA2 lies entirely within the borough, so Harrow validates every extension submitted from that district. |
| HA3 | Harrow Weald / Kenton / Wealdstone | 1930s semi-detached | Harrow Weald, Kenton and Wealdstone share the HA3 district and all fall to Harrow Council, which applies one set of Local Plan policies across the three. |
| HA7 | Stanmore | 1930s semi-detached, Edwardian villas | Stanmore occupies HA7 on its own, so a Harrow case officer handles every extension coming from a HA7 address. |
| HA5 | Pinner / Eastcote | Tudor fragments, Edwardian villas, 1930s semi-detached | HA5 straddles a boundary: Pinner falls to Harrow, Eastcote to Hillingdon, and the policies map settles which authority validates a given street. |
How does a house extension differ street by street across Harrow?
Harrow is the most uniformly interwar borough in London: 41.6% of its 96,860 dwellings were built between 1930 and 1939 and only 2.6% before 1900 (VOA council tax stock by build period at 31 March 2025, retrieved 1 September 2026). For an extension that uniformity is worth money, because the construction is predictable — cavity walls, concrete oversite, a rear plot deep enough for three metres without argument — and a builder who has done the street before can price it properly. The exceptions are the ones below: Harrow on the Hill and Pinner, where the buildings are older and the heritage controls are the tightest in the borough, and the Green Belt fringe at Stanmore and Harrow Weald.
- South Harrow (HA2) — a house extension in South Harrow starts from 1930s semi-detached, and South Harrow reads as 1930s semi-detached. 1930s semis here have the deepest rear gardens in the area in HA2. Harrow Council assesses South Harrow house extensions against its own South Harrow character guidance, and the location plan we draw for South Harrow references South Harrow Tube.
- Harrow on the Hill (HA1) — house extension work in Harrow on the Hill means georgian and victorian houses, listed buildings. Harrow on the Hill is Georgian and Victorian conservation hilltop. Georgian rear elevations here are usually the listed or locally listed face of the building throughout HA1. Harrow Council decides the Harrow on the Hill application, and Harrow School anchors the Harrow on the Hill location plan.
- Pinner (HA5) — a house extension in Pinner starts from tudor fragments, edwardian villas, 1930s semi-detached, and Pinner reads as Tudor and 1930s village. Edwardian plots here are wider in HA5. Harrow Council assesses Pinner house extensions against its own Pinner character guidance, and the location plan we draw for Pinner references Pinner High Street.
- Stanmore (HA7) — 1930s semi-detached, Green Belt fringe defines Stanmore, where 1930s semi-detached, edwardian villas set what a house extension can do. Edwardian plots here are wider across HA7. Any Stanmore house extension is validated by Harrow Council, with Stanmore Tube the usual Stanmore reference point on the site plan.
- Harrow Weald (HA3) — house extension work in Harrow Weald means 1930s semi-detached. Harrow Weald is 1930s semi-detached, Green Belt. 1930s semis here have the deepest rear gardens in the area throughout HA3. Harrow Council decides the Harrow Weald application, and Harrow Weald Common anchors the Harrow Weald location plan.
- Kenton (HA3) — 1930s semi-detached defines Kenton, where 1930s semi-detached set what a house extension can do. 1930s semis here have the deepest rear gardens in the area across HA3. Any Kenton house extension is validated by Harrow Council, with Kenton station the usual Kenton reference point on the site plan.
- Wealdstone (HA3) — 1930s semi-detached, post-war defines Wealdstone, where 1930s semi-detached, post-war flats set what a house extension can do. 1930s semis here have the deepest rear gardens in the area across HA3. Any Wealdstone house extension is validated by Harrow Council, with Wealdstone High Street the usual Wealdstone reference point on the site plan.
Does Harrow planning policy restrict a house extension?
The number behind the notification route: of those 1,194 larger extensions notified, 869 came back as prior approval not required — nobody objected, so no judgement was called for. The interesting figure is what happened to the remaining 325 where a neighbour did object and the council had to decide: 62 were granted and 263 refused, four in five turned down. So the Harrow notification is close to automatic while it is uncontested and very hard once it is not, and the objection period is therefore the whole game. A scheme that gives the adjoining owner a specific reason to write in — a flank wall on the boundary, an eaves height that overshadows a kitchen window — is worth reworking before it is submitted rather than after.
Where designation and the Green Belt take over: inside a conservation area — Harrow on the Hill, Pinner Village, Stanmore, Harrow Weald — article 2(3) removes the side-extension and cladding allowances outright, and the route becomes a £548 householder application in eight weeks rather than a £249 prior approval in six, with a heritage statement attached. On the Green Belt edge north of Stanmore the test changes again, to whether the addition is disproportionate to the original dwelling measured on volume. Elsewhere Harrow has made no borough-wide Article 4 direction, and 1,147 lawful development certificates were recorded here over the same three years — a count of applications, since the national return holds no granted-or-refused split for certificates in any English authority.
What happens when it goes to a decision: Harrow granted 77.8% of the 2,856 householder applications it decided, against 83.0% across London, and it delegates only 87.3% of its decisions to officers — the lowest share of the thirty-three, so an objected-to extension here is likelier than anywhere in the capital to be settled by councillors on a committee cycle that no statutory target accounts for. Against that, the appeal record is a genuine backstop: 277 of 698 householder appeals decided against Harrow between 2016 and 2026 were allowed, 39.7% against 35.6% for England (Planning Inspectorate casework, retrieved 1 September 2026). Appellants are the refused minority who chose to fight, so that is not an approval rate — but on a large sample it says a Harrow refusal is not the end of the matter.
What happens if a Harrow property sits on a boundary?
- Hillingdon — the western neighbour at Pinner and Northwood, and the closest match in stock: it is the third least pre-1900 borough in London at 1.8%, just ahead of Harrow itself, so the houses either side of the line are effectively the same houses. What differs is the committee habit. Harrow refers more decisions to members than any London borough; Hillingdon does not.
- Barnet — the eastern edge at Edgware and Stanmore. Barnet refuses larger-extension notifications at 37.6% overall against Harrow's 22.0%, so an identical rear extension can face a materially different notification risk depending on which side of the Edgware boundary the plot sits.
- Brent — the southern edge at Kenton, Queensbury and Wembley. Considerably more mixed stock than Harrow, with terraces and flatted conversions alongside the semis, and a heavier enforcement record: 399 enforcement notices issued over the same three years against Harrow's 65 (MHCLG planning statistics, retrieved 1 September 2026).
Sources for this Harrow house extension page: borough policies maps and
conservation-area registers (2025–26), Royal Mail postcode district boundaries,
the Town and Country Planning (Fees for Applications) (England) Regulations, the Town
and Country Planning (General Permitted Development) (England) Order 2015 as amended,
and the Party Wall etc. Act 1996.
