Guide · 31 October 2026

Building costs and house prices in Harrow

Harrow is classic outer-London suburbia: Metroland streets of 1920s and 1930s semis around Pinner, Rayners Lane, Kenton and Stanmore, an older hilltop village round Harrow School, and Green Belt fields along the northern edge. Because so many homes here are family houses with gardens, the question of whether to move or extend comes up in almost every street, and the answer is closer here than in most of inner London.

The Census 2021 accommodation-type table (TS044, via Nomis) counted 89,642 households in Harrow. 37.1% lived in a semi-detached house, about twice London’s 18.6%. Terraced houses held 16.9% and detached houses 11.3%, while 26.2% lived in purpose-built flats and 6.4% in flats in a converted or shared house. In all, about 65% of households here live in a whole house, against 46% across London. Harrow is also a borough of owners: the Census tenure table (TS054) found 58.8% of households owned their home outright or with a mortgage, against 45.2% in London as a whole.

What homes cost in Harrow in August 2026

The UK House Price Index from HM Land Registry and the ONS, published on 21 October 2026, puts Harrow’s average price at £532,173 for August 2026. The figures are provisional. Prices were 2.5% lower than a year earlier, though they rose 0.6% on July. London’s average was £554,430, down 2.2%, so a typical Harrow home costs about £22,000 less than the London figure and the borough sits 19th of 33 on price. Harrow’s high point was £561,854 in August 2022, about 5% above today. First-time buyers paid £437,780 on average, and people who had owned before paid £662,482.

Houses barely moved while flats fell by about a twentieth. Harrow’s terraces, semis and flats are all cheaper than London’s type averages, but its detached houses are slightly dearer. Land Registry recorded 77 sales of all types in the borough in June 2026, the latest month with a count. The figures come from the HPI full file and the ONS local house prices page for Harrow. The HPI models type averages rather than simply averaging the homes that sold, and the figures will be revised, so they describe the borough as a whole and can’t value a particular house.

Move to a bigger home, or build?

On August’s averages, moving from a flat to a terraced house meant finding about £225,000 more. The step from a terrace to a semi was much smaller, about £127,500, and from a semi to a detached house about £545,000. Stamp duty, legal fees, surveys and removals come on top of each of those.

No one publishes building costs for Harrow alone, so the figures below are national. Checkatrade’s house extension cost guide (updated June 2026, with estimates checked by RICS and CIOB members) gives these UK ranges, excluding VAT:

• £1,800–£3,000 per m² for a finished single-storey extension
• £30,000–£42,000 for a 15m² rear extension to a mid-terrace house
• £108,000–£180,000 for a 60m² two-storey extension
• £50,000–£95,000 for a 30m² loft conversion
• £100,000–£160,000 for a 40m² basement
• £8,500–£20,000 for a 15m² garage conversion
• £350–£950 for architectural drawings

Checkatrade says its figures don’t reflect location and that London and the South East usually cost more, so expect local quotes above these ranges, and add VAT. That matters in Harrow, because the terrace-to-semi gap is small enough that a large two-storey extension, priced at London rates, can come close to it once moving costs are added on the other side. A loft or a single-storey rear extension still costs well under the £127,500 gap, and many interwar semis here have a side garage that can be converted for less than either. The semi-to-detached step is a different matter: at more than half a million pounds, building is far cheaper than buying. Flat owners have fewer options, because flats have no permitted development rights and leaseholders usually need the freeholder’s written consent too. Whichever way you lean, don’t assume the building cost comes back when you sell; judge the work on whether it gives you the space you need.

Permitted development: the national rules

For houses, the Planning Portal’s extension guidance sets out what can be built without an application. A single-storey rear extension can project 3 metres from the original back wall of a terrace or semi, or 4 metres for a detached house. Under the prior approval scheme those limits rise to 6 and 8 metres, with a maximum height of 4 metres. Harrow’s prior approval guidance notes explain that the council writes to the adjoining neighbours, who have 21 days to comment, and decides on the effect on them. A single-storey side extension can be up to 4 metres high and half the width of the original house. The loft conversion guidance allows up to 40 cubic metres of extra roof space on a terrace and 50 cubic metres on other houses, with nothing built out from the roof slope facing the road.

These rights are narrower in conservation areas, where side extensions, rear extensions of more than one storey and dormers need planning permission, and on streets covered by Article 4 directions (see below). If your plans do fit, a Lawful Development Certificate from the council gives you proof to show a future buyer. Building regulations approval is needed either way.

How Harrow judges extensions

Applications are decided against the London Plan and the Harrow Local Plan 2021–2041, which the council adopted in March 2026 after an inspector found it sound. The detailed design rules for houses are in the Residential Design Guide SPD, adopted in December 2010. Its main points are:

• Rear extensions. On a semi or terrace, 3 metres deep is the usual starting point, and 4 metres on a detached house. Anything deeper must be set in from the side boundary by twice the extra depth, which the guide calls the “two for one” rule. Height on the boundary should be no more than 3 metres for a flat roof, or 3.5 metres at the middle of a pitched one, and a 45-degree test protects neighbours’ windows.
• Side extensions. A single-storey side extension up to the boundary is normally acceptable. A two-storey or first-floor side extension should have a pitched roof, preferably hipped, and the upper floor should step back at least 1 metre from the front so that a pair of semis doesn’t read as a terrace.
• Lofts. For semis and end terraces the guide’s preferred design is a hip-to-gable change with a rear dormer. Dormers should be clearly smaller than the roof, sit at least 1 metre up from the eaves, 1 metre in from the gable and half a metre in from the party wall, and stay below the ridge.
• Conservatories. One built within 3 metres of a boundary should have a solid wall on that side.

The new Local Plan also matters if you’re thinking of splitting a house. Policy HO2 only allows a family house to be converted into flats when it is larger than 130m² as originally built (or can acceptably be extended to that size) and one of the new homes is a family-sized flat with its own garden. In the north of the borough, land in the Green Belt has much tighter limits on extending.

Conservation areas and Article 4

The council’s conservation pages say Harrow has 30 conservation areas, grouped into four sets, each with its own appraisals and management strategies: Harrow on the Hill (adopted May 2008), Pinner (December 2009), Stanmore and Edgware (December 2013) and Harrow Weald (February 2016). Several cover the interwar estates that give the borough its character, such as Canons Park Estate, Pinner Hill Estate and Rayners Lane, as well as older centres like Pinner High Street and Harrow on the Hill Village.

Article 4 directions apply in 16 of those areas, including Canons Park Estate, Stanmore Hill, Little Common, Kerry Avenue, Mount Park Estate, Harrow Park, Roxborough Park and The Grove, Pinner High Street, Pinner Hill Estate and West Towers, and to some buildings in Brookshill Drive and Grimsdyke Estate. They take away some permitted development rights, often for details such as windows, doors and roofs, so work that would be automatic elsewhere needs an application. The Article 4 page sends you to each area’s own page for the details, and the council also publishes a residential planning guide for conservation areas. Check your address before you commission drawings.

Basements

The Planning Portal says a new basement is likely to need planning permission. Harrow’s Local Plan policy GR4A supports basements only as extra space for the house above, reached from inside it, and not as separate flats. A basement shouldn’t take up more than half of the outdoor space on the site or reach into the back garden further than half the depth of the house, measured from its rear wall. It mustn’t harm neighbours, worsen flooding or groundwater problems, threaten ground stability or damage tree roots, and lightwells and railings at the front should be kept modest. Check your address on the government flood map for planning early on.

Party walls

With so many semis and terraces, most lofts, rear extensions and basements in Harrow touch the Party Wall etc. Act 1996. A hip-to-gable loft on a semi, steel beams set into the shared wall, building on the boundary or digging near a neighbour’s foundations all mean serving notice before work starts. GOV.UK’s party wall guidance explains the notices and the time limits. If a neighbour doesn’t consent, a single agreed surveyor, or one for each side, draws up an award that sets out how the work will be done, and the person building normally pays. A friendly conversation before any paperwork arrives usually saves time and money.

How to avoid a rogue builder

Be wary of anyone who knocks uninvited, pushes you to decide on the spot, asks for a large cash payment up front or gives one round figure with no breakdown. A sound firm puts an itemised quote and a written contract in front of you, takes staged payments as the work progresses, shows its insurance and past jobs, and makes clear who handles building regulations sign-off.

Look for registered firms through TrustMark, work through Citizens Advice’s checklist before building work, and report rogue traders to your local Trading Standards office.

Figures change: get quotes before you decide

House prices here are UK HPI figures for August 2026, published on 21 October 2026. They are provisional and will be revised, and September’s figures are due on 18 November 2026. Building costs are Checkatrade’s national ranges, last updated in June 2026; they aren’t local prices, and London work usually costs more. Household and tenure figures come from Census 2021, taken on 21 March 2021. Planning details come from the Harrow Local Plan 2021–2041 (adopted March 2026), the 2010 Residential Design Guide and the council’s web pages as they stood when we checked them on 31 October 2026. Check the rules for your own address, and get at least three itemised quotes based on proper drawings before choosing between building and moving.

Sources: UK House Price Index England: August 2026 (GOV.UK); UK HPI data downloads, August 2026 (full file, Harrow E09000015); ONS local house prices: Harrow; Census 2021 TS044 accommodation type (Nomis); Census 2021 TS054 tenure (Nomis); Checkatrade: house extension costs; Planning Portal: extensions; Planning Portal: loft conversions; Planning Portal: basements; Harrow Local Plan 2021–2041 (PDF); Harrow Council: Residential Design Guide SPD; Harrow Council: prior approval guidance notes; Harrow Council: conservation areas; Harrow Council: Article 4 directions; Harrow Council: conservation area residential planning guides (PDF); GOV.UK flood map for planning; GOV.UK: Party Wall etc. Act guidance; TrustMark; Citizens Advice: before building work; GOV.UK: find local Trading Standards. Data checked 31 October 2026.