Hampstead Architecture and Planning

Old Oak and Park Royal Development Corporation

House Extension Architects in Harlesden

Harlesden is the one place in this part of London where the first question is not which borough you are in, but whether you are in a borough at all. The checked research point returns the Old Oak and Park Royal Development Corporation — a Mayoral Development Corporation that is itself the local planning authority for its designated area — with Ealing and Brent also within 1,400 metres. Establishing which applies to your address is the whole starting point.

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The short answer

What do you need to know before extending in Harlesden?

Which authority determines your application, and it is genuinely not obvious. The checked Harlesden research point falls inside the Old Oak and Park Royal Development Corporation — a Mayoral Development Corporation acting as the local planning authority for its own area — while Ealing and Brent are also within 1,400 metres, and a scheme of delegation returns some functions to Brent. Settle that on the official boundary layer first; the extension options themselves are the standard NW10 three.

Planning at a glance

Harlesden

Planning at a glance in Harlesden
Local planning authorityOld Oak and Park Royal Development Corporation
Likely approval routeHouseholder planning application or permitted development, depending on conservation status, listing and property type — confirmed for the exact address before drawings begin
Statutory determination time8 weeks for most householder applications (13 weeks for major applications)
Typical drawing package£2,250–£3,250 inc. VAT, fixed fee. Find my package →

The practical consequence of the Harlesden jurisdiction question is that ordinary shortcuts do not work here. A neighbour’s recent approval, a borough guidance note found online, an assumption about fees or about the Community Infrastructure Levy — each of those is only reliable once you know which authority actually determines applications at your address. The development corporation publishes its own local plan and its own validation requirements, and while the scheme of delegation means Brent handles certain functions within the Brent part of the corporation’s area, that arrangement does not make the two interchangeable. We resolve it by plotting the address on the official boundary layer before any design work begins, and recording the result.

Once jurisdiction is settled the extension options are much like the rest of NW10, and they depend on house type. The Victorian and Edwardian terraces around the town centre have a rear closet wing and a narrow side passage, so the transformative move is usually the side return infill rather than extending straight back — it adds width where the house is narrowest. On the interwar semis further out there is a side gap and a deeper garden, which opens up side extensions and wraparounds that a terrace cannot support, and gives the Class A allowances more room to work with.

Harlesden also rewards a phased approach more than most areas, because the housing stock is comparatively affordable and owners are frequently improving a house over years rather than in one campaign. Phasing works well provided the design is produced as a whole: the structural openings, the drainage runs, the service routes and the fire strategy are all set out once, and each phase is built to a plan that anticipates the next. Phasing badly — extending the rear now and discovering later that the loft conversion needs the ground-floor stair enclosure rebuilding — is what makes two projects cost more than one.

Drainage and party wall work are the same here as elsewhere in the terraced belt, and they still need proper attention. The side passage on a terrace carries the rainwater and foul runs, and building over a public sewer needs a build-over agreement with Thames Water and a foundation design to match. Excavating within three metres of a neighbour’s structure below their foundation level, or building along the line of junction, requires notice under the Party Wall etc. Act 1996, and on a terrace that usually means two adjoining owners.

Local analysis

What matters in Harlesden

Working out who determines your Harlesden application

This is the check that has to happen first, and it is genuinely more involved than in a normal borough. The Old Oak and Park Royal Development Corporation was established by the Mayor of London to secure the regeneration of the Old Oak opportunity area, and within its designated boundary it is the local planning authority: it produces the local plan, sets the policies against which applications are judged and determines those applications. That boundary takes in land across Ealing, Brent and Hammersmith & Fulham, and the checked Harlesden research point falls inside it. A scheme of delegation between the corporation and Brent formalises the delegation of certain planning functions back to Brent in the parts of the corporation’s area that lie within the Brent boundary, which is why homeowners sometimes find themselves dealing with Brent officers on a corporation matter. None of that changes the underlying position: the policies that apply, the plan your scheme is assessed against and the conservation designations that bind depend on where the boundary falls relative to your address. We plot it on the official layer, record the answer, and then select guidance and precedents from that authority alone.

Phasing a Harlesden project so the second stage is cheaper

Improving a house over several years is a sensible way to spread cost, and it works well if the whole design exists from the outset even though only part of it is built. The elements that have to be decided once, and early, are the ones that are expensive to revisit. Structural openings should be sized for the final arrangement, so a beam installed for the rear extension already accommodates the loads a later loft conversion will add. Drainage should be laid out for where the bathrooms will eventually be, not only where they are now, because adding a soil run through a finished house is disruptive and sometimes impossible without boxing. Service routes — the vertical paths for extract ducts, cabling and pipework — need reserving before ceilings are closed up. And the fire strategy has to anticipate the final number of storeys, because a loft conversion turns the house into a three-storey building and requires a protected escape route through the floors below; building an open-plan ground floor now and reinstating separation later means paying twice. We produce the full design and then a phasing plan that says explicitly what is built when and what is left ready, so each stage lands on work already done rather than undoing it.

Thinking about a project in Harlesden?

Send the address and a short description. We will confirm the likely approval route and the drawings your project needs.

Book an Architectural ConsultationSee fixed-fee packagesor call 020 8054 8756

Local planning

Planning & conservation context

The Old Oak and Park Royal Development Corporation is a Mayoral Development Corporation established by the Mayor of London and is the local planning authority for its designated area, which spans land in Ealing, Brent and Hammersmith & Fulham. Inside that boundary it produces the local plan and determines planning applications, and a scheme of delegation returns certain planning functions to Brent within the Brent part of the area.

Outside that boundary, Brent or Ealing is the authority depending on the address. Brent covers all of its residential conservation areas with Article 4 directions, and confirmed a borough-wide Article 4 direction in February 2022 removing the permitted development right to convert a dwellinghouse into a small house in multiple occupation.

House extensions run through Schedule 2, Part 1, Class A of the General Permitted Development Order, with reduced allowances on designated land and no householder rights at all for flats. Which authority determines any application that is needed depends entirely on where the boundary falls relative to your address.

The conservation records within 1,400 metres of the Harlesden point split across authorities: Harlesden and its extension, and Kensal Rise, are Brent designations; Canalside north-east part, the Grand Union Canal and Old Oak Lane fall under the Old Oak and Park Royal Development Corporation.

That split matters because the two sets of records are administered by different bodies with different local plans and different guidance. A canal-side designation under the development corporation says nothing about a Brent address a few streets away, and the reverse holds equally.

Whether a particular house is covered is settled by plotting the address on the responsible authority’s current conservation mapping. A record appearing within the search radius is a prompt to check, not evidence of designation.

Illustrative study — street elevation

Services

Architectural services in Harlesden

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Nearby

Nearby House Extension Architects pages

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Architecture in Harlesden: FAQs

Which planning authority covers Harlesden?

It depends on the address. The checked research point returns the Old Oak and Park Royal Development Corporation, which is a Mayoral Development Corporation and the local planning authority for its designated area spanning land in Ealing, Brent and Hammersmith & Fulham. Ealing and Brent also fall within 1,400 metres. A scheme of delegation returns certain functions to Brent within the Brent part of the corporation’s area, so the position needs establishing precisely.

What is a Mayoral Development Corporation?

It is a body established by the Mayor of London to lead regeneration in a defined area, and within that area it takes on the local planning authority role: producing a local plan, setting policy and determining planning applications. The Old Oak and Park Royal Development Corporation covers the Old Oak opportunity area and adjoining land. It is not a borough, so borough guidance and borough precedents do not automatically apply inside its boundary.

Do I need planning permission to extend a house in Harlesden?

It depends on the address and its designation. On an undesignated house, Class A permitted development can cover a single-storey rear extension within its limits. On designated land the allowances reduce and side extensions drop out. Brent covers all of its residential conservation areas with Article 4 directions. A property divided into flats has no householder rights at all. The check runs against whichever authority determines applications at your address.

What can I build on a Harlesden terrace?

The same three options as elsewhere in NW10: a rear extension, a side return infill where a passage exists, or a wraparound combining both. On these terraces the side return usually improves the house more, because it adds width where a Victorian plan is narrowest. A wraparound gives the largest open ground floor but needs a full application and engages both boundaries for party wall purposes.

Can I extend an interwar semi in Harlesden?

Usually with more scope than a terrace. The semis on the outer streets typically have a side gap and a deeper garden, which opens up side extensions and wraparounds and gives the Class A allowances more room to work with. Where a side extension is proposed, keeping a visible gap to the boundary and setting the upper level back from the front elevation is what makes it acceptable on a street of matched pairs.

Can I do the work in phases?

Yes, and it suits many projects here — but the whole design should exist from the start even if only part is built. Size structural openings for the final arrangement, lay drainage for where bathrooms will eventually be, reserve vertical service routes before ceilings close, and plan the fire strategy for the final number of storeys. Otherwise a later loft conversion means rebuilding the stair enclosure on a ground floor you have just finished.

Do I need a build-over agreement for a rear extension?

Where the extension sits over or within three metres of a public sewer, yes — a build-over agreement with Thames Water is required and the foundations are designed around the line, whether deepened, offset or bridged. On terraces the runs often pass through the side passage and are frequently not where the deeds imply. A drainage survey at feasibility keeps that cost inside the budget rather than outside it.

What does a house extension architect in Harlesden charge?

We publish fixed-fee packages rather than quoting a percentage before seeing the property, starting with a route review credited against the design work if you proceed. The stages are feasibility and consent strategy, then planning or certificate drawings, then building regulations and tender information. Structural engineering, drainage surveys, party wall surveyors, application fees and building control charges sit outside the architectural fee.

How much does a house extension cost in Harlesden?

Fees for house extensions in Harlesden follow scope, consent complexity and whether freeholder or specialist input is required. Published packages for related work start from around £2,250–£3,250 once scope is clear; council, engineer and freeholder surveyor fees are usually separate. See our packages page for current fixed-fee bands — we confirm a fee only after the address and proposed work are understood.

Discuss a house extension in Harlesden

Send your Harlesden address, photographs of the rear elevation and any side passage, and a note of what you want to gain. We will establish which authority determines your application, check the conservation position, and set out the realistic options and the fixed-fee stage that follows.

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