The short answer for Mill Hill
Planning authority
London Borough of Barnet
Postcode district
NW7
Conservation area
152 hectares (2008)
Houses inside it
About 260 (2008)
Not sure you are in Mill Hill? Check your area
Yes, we cover Mill Hill. Barnet's conservation area here is 152 hectares along The Ridgeway, yet its own appraisal counts only about 260 houses in it, alongside schools, convents and other large institutions, so most Mill Hill addresses sit outside it. Inside it, Barnet's Article 4 directions control doors, windows, roofs, porches, painting, satellite dishes and boundaries, and none of the documents names air conditioning or a heat pump. What does apply is the national rule: a unit cannot go on a wall that fronts a highway, and permitted development does not apply to a listed building, of which Historic England records 46 within the boundary. Green Belt around the village does not by itself remove the right to fit an outdoor unit. All of NW7 is Barnet, so one council decides.
Wall mounted systems start from £1,995 fully installed, and the price does not change with the postcode.
Free survey
Get a written quote in Mill Hill
- We call you back, or email you if we cannot get through.
- We come and do a free survey at your address.
- You get one price in writing, agreed before any work starts.
The survey is free, and nothing is booked before you have the price in writing.
Big, long and thin, and mostly not houses
Barnet's adopted character appraisal for the Mill Hill Conservation Area, dated April 2008, puts it at approximately 152 hectares, strung out for a mile along the high ground of The Ridgeway. It counts approximately 260 houses, brick and weatherboard cottages, among a large number of institutional buildings.
The institutions are most of the story. The appraisal names Mill Hill School, Belmont School and The Mount as the large schools, and religious orders including the Priory, St Vincent de Paul Convent and St Josephs.
For scale, the Mill Hill ward alone had 6,921 households at the 2021 Census. So most Mill Hill addresses are not in the conservation area, and for them the questions are the ordinary ones: which wall, how far the pipe runs, where the water goes.
The Article 4 directions here are about the front of a house, not an outdoor unit
Barnet's appraisal lists what its Article 4 direction controls: replacement windows and doors, roof extensions and roof lights, new wall materials and painting, small extensions such as porches and outbuildings, oil tanks, satellite dishes, boundaries, hard standings and most new gates, fences and walls. It applies to selected areas, namely the area around the Old Forge, The Ridgeway, Milespit Hill, Wills Grove and Winterstoke Gardens. An air conditioning unit or a heat pump is not on that list, and it is not in the direction documents Barnet publishes, which run from 1970 to 2008. That does not leave the area unregulated. It means the rule that matters is the national one in the next section.
What the housing actually is
Households by accommodation type at the 2021 Census, for the Barnet ward called Mill Hill. It is a ward of houses more than flats: 2,367 semi detached and 1,303 detached, against 1,849 in purpose built blocks.
| Ward | Households | Detached | Semi-detached | Terraced | Purpose built flats |
|---|---|---|---|---|---|
| Mill Hill | 6,921 | 1,303 | 2,367 | 757 | 1,849 |
ONS Census 2021, table TS044, accommodation type by household, as at 21 March 2021, on the 2022 ward boundaries. Only the categories ONS publishes are shown; the remaining households are in converted or shared houses, other converted buildings, commercial buildings or temporary structures. The NW7 postcode district also reaches into neighbouring wards, including Edgware and Totteridge & Woodside, so this is the ward called Mill Hill and not every NW7 address.
What applies to you
Your home is outside the conservation area
Permitted development applies, with the national limits. The unit must not go on a wall that fronts a highway above ground floor level, and a reversible system that heats as well as cools keeps it inside the rule. Side or rear elevation, pipe run inside three metres, usually one visit.
Your home is inside the conservation area
The national rule tightens: no unit on a wall or roof that fronts a highway, and none nearer a bounding highway than the nearest part of the house. Where the house is close to the road, that points to the rear. Barnet's Article 4 schedule names individual properties, so check yours as well.
Your home is listed
Permitted development does not apply to a listed building or to land within its curtilage. Historic England records 46 entries within the conservation area boundary, 45 Grade II and one Grade II star, Holcombe House. They include rows of cottages as well as schools. Speak to Barnet's heritage team before anything else.
You live in a flat
At the 2021 Census, 1,849 households in the Mill Hill ward lived in purpose built blocks, and Barnet's Local Plan identifies capacity for about 2,280 additional homes at Mill Hill East. In a block, the freeholder's written consent comes first. Above the first floor, expect a condensate pump, priced as a line on the quote.
You run a school, an institution or a business
Class G covers dwellinghouses and blocks of flats, and a school or a convent is neither, so an application to Barnet is likely. Our commercial page covers how we handle those, and we will give you the unit dimensions and position for the form.
Green Belt, and checking your own address
Barnet's Local Plan, adopted in March 2025, sends development in the Green Belt to national policy in its policy ECC06, and says the Mill Hill East area includes both Green Belt and the conservation area. That matters for extensions and outbuildings. Class G, the national rule for air conditioning, does not list Green Belt among the reasons to lose permitted development.
Barnet decides every NW7 address, so there is no second council to check. Its schedule of Article 4 properties, dated January 1991, names individual properties on Hammers Lane, High Street, Highwood Hill, Lawrence Street, Milespit Hill, The Ridgeway, Winterstoke Gardens and Wills Grove. If your road is there, we check your address against it at the survey.
Your planning authority is London Borough of Barnet
They decide any application for your address, and they publish the conservation area maps you would need to check it. We do not submit applications on your behalf, but we will tell you at the survey if one is likely and give you the unit dimensions and position to put on the form.
London Borough of Barnet conservation areasWhat we would fit in Mill Hill
Every price is fully installed and includes the unit, the installation, the electrical work and VAT.
Nearby
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Instant estimate
Start with the room, not the designation.
For most Mill Hill addresses the question is which wall the unit goes on, not the conservation area. Size your room in about a minute, and the free survey confirms the position, with a written quote before any work starts.