TL;DR — Key Facts
- Build cost per sqm: £2,200 – £3,200 (outer London borough)
- Council planning fee: £548 (householder application, 2026)
- Typical projects: rear extension, side-return, two-storey, wraparound
- Conservation areas: 29 designated areas in Harrow
- Permitted Development: Generally available outside conservation areas
What does Harrow do with an extension application?
Harrow approved 77.8% of its householder applications and sent an unusually large share to committee: only 87.3% of decisions were delegated to officers, the lowest rate in London. On the 6m route it granted 62 prior approvals against 263 refusals, so the notification is far from a formality here.
| Measure | Harrow | London |
|---|---|---|
| Decisions made under delegated powers | 87.3% | 97.0% |
| Householder approval rate | 77.8% | 83.0% |
| Householder decisions made | 2,856 | 77,680 |
| Larger-extension prior approvals granted | 62 | 19.1% of those determined |
| Lawful development certificates issued | 1,147 | — |
| Householder appeals allowed after refusal | 39.7% of 698 | 40.8% of 13,067 |
Sources: MHCLG District Planning Application Statistics (PS1 and PS2), decisions made in the twelve quarters to March 2026, and Planning Inspectorate casework decided 2016–2026. Open Government Licence v3.0, retrieved 1 September 2026. Harrow also recorded 328 larger-extension notifications where prior approval was not required, so the full picture of that route is 62 granted, 263 refused and 328 waved through. Build costs on this page are our own 2026 London range, not a published statistic.
What does a Harrow extension cost in 2026?
That is the money. Once the number makes sense, house extensions in Harrow walks through the drawings and the submission itself.
Build costs for a Harrow extension sit in our outer-London band of £2,200 to £3,200 per square metre, covering construction only — not professional fees, not planning fees, not VAT. Harrow has the most consistent housing stock we work on anywhere in London. Of its 96,860 dwellings, 41.6% were built between 1930 and 1939 and just 2.6% predate 1900, against London figures of 16.0% and 20.9% (VOA council tax stock at 31 March 2025, published via the London Datastore). Metroland, essentially. The practical consequence is that a Harrow quote carries fewer unknowns than the same room priced in an inner borough: cavity walls, a roof of known age and construction, a side passage to work down, and a garden long enough to build into without an argument about daylight.
Most of Harrow is interwar semi where the cost is a straightforward function of span and glazing. Harrow on the Hill is the exception and it is a large one: heritage controls there are the strictest in the borough, and matched materials plus the supporting heritage evidence add cost before a spade goes in the ground. Check which of the two you are in before taking a builder's figure seriously.
Typical extension costs in Harrow (build only)
Drawings are the next line, and Harrow gives an unusual reason to invest in the written case as well as the plans. This council delegates 87.3% of its decisions to officers — the lowest delegation rate of the 33 London planning authorities, against 97.0% pooled across the capital (MHCLG planning application statistics, twelve quarters to March 2026, retrieved 1 September 2026). Roughly one application in eight goes to a committee rather than to a case officer, which is more than four times the London norm. Committee members read a set once, cold, in a room, with no chance to ask what a hatch means. That changes what the drawings have to do. Our fees are fixed in advance and run about 30% below typical London architect rates.
What moves the price on a 1930s Harrow semi?
Harrow delegates only 87.3% of its planning decisions to officers (MHCLG planning statistics, retrieved 1 September 2026), the lowest share in London - roughly one in eight is taken by councillors in public. For an extension that means neighbour objection carries more weight here than the policy documents alone suggest, and it can add a committee cycle to the programme. The stock itself is interwar semi with side access, so the build side is straightforward.
Conservation areas
Designation changes the route before it changes the specification. On article 2(3) land, permitted development no longer covers an extension beyond a side elevation, a rear extension of more than one storey, or cladding and render, so a £548 householder application takes the place of the certificate. Harrow’s designations sit mostly over the older village cores and the planned estates rather than over the interwar grid: the Canons Park Estate covers 63.5 hectares, Little Common at Stanmore 32.5, Sudbury Hill at Harrow on the Hill 30.8 and the Pinner Hill Estate 29.5 (planning.data.gov.uk conservation area register, retrieved 1 September 2026). The borough holds 295 listed buildings and grants consent for work to them readily — 64 of the 67 applications to alter or extend were approved, the fifth-highest rate of the 26 boroughs where the figure can be calculated.
Property type
One house type dominates so heavily here that it is worth naming what it means for the work. The Harrow semi — bay front, hipped roof, side passage, built somewhere between 1930 and 1939 — takes a two-storey rear extension more comfortably than almost any other London stock, because the pairs were laid out with space between them and the upper floor can be set back from the flank boundary without losing a usable room behind it. The Edwardian villas of Harrow on the Hill and Pinner are a smaller and far more varied group, where the answer comes off the plot rather than out of a pattern book. Detached houses buy the most freedom and the largest bill: the rate per square metre barely shifts, but the room you have in mind is bigger.
Access and logistics
A 1930s Harrow semi almost always has a side passage wide enough for a barrow and usually for a mini excavator, which keeps the groundworks cost conventional. Harrow on the Hill and Pinner's Edwardian villas are more variable. Where the property backs onto Green Belt land in Stanmore or Harrow Weald, access is generally excellent and the policy test is the difficulty instead.
Our drawing fees for Harrow extensions
No hourly billing and no variations. The fee is fixed before we start and covers everything a valid submission to Harrow Council needs, plus the building regulations package if you take that as well. If a refusal does come, Harrow refusals stand up rather less reliably than the headline approval rate suggests: the Planning Inspectorate decided 698 Harrow householder appeals between 2016 and 2026 and allowed 277 of them, 39.7% against 35.6% across England (Planning Inspectorate appeal casework, retrieved 1 September 2026). That is a statement about how this borough’s refusals fare on review, not a forecast for a first application.
Drawing fees for extensions in Harrow
Each package opens with a measured survey of the house as it stands and delivers existing and proposed plans and elevations, a site plan, and a design and access statement where the application needs one. Our MCIAT chartered technologists carry the project from survey to submission and handle everything that follows — which in this borough is more likely than elsewhere to mean a committee report rather than an email from a case officer. We draw for that possibility from the start rather than reacting to it in week six.
Harrow work runs across HA1, HA2, HA3, HA5, HA7 and HA8 — the town centre out to Wealdstone, Rayners Lane, Pinner, Stanmore and the Edgware edge.
Planning or permitted development in Harrow?
An unusually large share of Harrow extensions need no planning application at all, and the volume proves the point better than any policy statement could. Under permitted development a single-storey rear extension can project 3 metres from the original rear wall on a terrace or a semi and 4 metres on a detached house, doubling to 6 and 8 metres through the neighbour consultation scheme. Harrow determined 1,194 of those larger notifications in the three years to March 2026 — more than twice Croydon’s 529 and more than three times Bromley’s 377 — and found that 869 of them needed no prior approval at all, refusing 263, a 22.0% refusal share against 24.9% pooled across London (MHCLG planning application statistics, PS1, retrieved 1 September 2026). This is a borough where owners know the deep rear extension route exists and use it.
Harrow has 29 conservation areas and no borough-wide Article 4 Direction. On the interwar semis that make up most of the borough, losing the side-extension right is the substantive restriction. On Harrow on the Hill the materials rule dominates, and the heritage controls there are stricter than anywhere else in the borough by a considerable margin.
Two-storey extensions almost always need an application, and in Harrow there is a procedural point worth knowing before you make one. This council delegates only 87.3% of its decisions to officers (MHCLG planning statistics, retrieved 1 September 2026), the lowest share in London, so a first-floor addition that attracts neighbour objection has a realistic chance of being decided by councillors in public rather than by a case officer.
The council granted 1,147 certificates of lawfulness across three years (MHCLG planning statistics, retrieved 1 September 2026). In a borough where more decisions than usual are made in public, taking work outside the application process altogether has an obvious appeal - and the certificate is how that position is recorded. It is decided by officers on the facts, not by members on the merits.
Frequently asked questions
How much does a house extension cost in Harrow?
Harrow build rates run £2,200–£3,200 the square metre: outer-London money, not inner. A single-storey rear addition of 15 to 20 square metres therefore lands between £33,000 and £64,000 before anyone draws a line. Design fees, structural calculations, a surveyor's award if you are building against a shared boundary, and the council's own charge all sit above that line. The householder application fee in England is £548. For scale: Harrow's median sale price in 2025 was £550,000 across 1,665 transactions, unchanged on 2024, with the lower quartile at £376,500. Source: HM Land Registry Price Paid Data.
Can I extend under Permitted Development in Harrow?
Harrow householders reach for the lawful development certificate more often than the extension-cost guides suggest. The council recorded 1,147 certificate of lawful development decisions between April 2023 and March 2026, against 2,856 householder planning applications over the same three years — roughly one certificate for every two and a half applications. No grant rate exists for certificates, in Harrow or any other English authority, because MHCLG does not collect one; the count is the whole of what the data supports. Most single-storey rear extensions do qualify as permitted development, at three metres for a semi or terrace and four for a detached house, rising to six and eight metres through prior approval under the neighbour consultation scheme. Those rights are cut back across Harrow's 29 conservation areas. The fee for a lawful development certificate is £274, and a Lawful Development Certificate is exactly what a conveyancer asks to see when the house changes hands years later. Source: MHCLG live planning statistics.
How long does a house extension take in Harrow?
Five to eight months is the realistic span in Harrow, and planning is rarely the long pole. Two to three weeks covers the measured survey and the drawing set. Of 2,121 householder applications Harrow decided outside the fast-track route between April 2023 and March 2026, 1,931 were determined inside eight weeks, so the statutory clock generally holds. Building regulations approval runs alongside the planning stage rather than after it. Construction is the variable: eight to sixteen weeks, longer if the ground turns out worse than the trial hole suggested. Source: MHCLG live planning statistics.
Do I need a Party Wall Agreement for an extension in Harrow?
One adjoining owner on a Harrow semi. The Party Wall etc. Act 1996 bites when the new foundations go within three metres of theirs at a lower depth, or when the extension is built off the boundary line. Serve the notice early: given that a contested scheme here may go to committee, the party wall process and the planning process can run in parallel rather than in sequence.
What experience do you have with extensions in Harrow?
We prepare extension drawings for properties across all Harrow postcodes (HA1, HA2, HA3, HA5, HA7, HA8). Our MCIAT chartered architectural technologists understand Harrow Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
