House extension in Mill Hill

Connect the added space to the existing home.

Start with the house extension brief

Begin with how the new space will meet the old rooms, garden and circulation. A measured brief should record boundaries, levels, existing drains, neighbouring windows and the parts of the building you intend to keep.

Develop daylight, structure and services alongside the floor plan. Agree which openings and finishes are included, how the existing home will be protected during construction and which decisions need surveys or approval before a quotation.

The local building context

Barnet’s Mill Hill Conservation Area Character Appraisal Statement, adopted in April 2008, places the conservation area towards the north-west corner of the borough: about 152 hectares, with institutional buildings and houses strung out for a mile along the high ground of The Ridgeway, looking over Green Belt countryside from Burtonhole Lane to the south-east and Marsh Lane to the north. It counts approximately 260 houses, brick and weatherboard cottages typical of rural Middlesex among a large number of rambling institutional buildings, with three clusters of smaller houses and cottages along The Ridgeway.

Mill Hill Village, a cluster of small vernacular dwellings at the top of Milespit Hill, is mainly two-storey late Victorian development in brick under slate or tile, or render under tile, behind white-painted picket fences. Wills Grove and Winterstoke Gardens have early twentieth-century Arts and Crafts houses in red brick and tile, with roughcast or brick walls, steep roofs, timber casements, prominent chimneys and hedged boundaries. By 2008 some former Mill Hill School boarding and staff houses on Wills Grove were private houses and apartments, and the appraisal records John Laing’s 1933 Sunnyfields Estate where Lawrence Street meets Highwood Hill.

The council, the conservation boundary, any listing and the consent route all depend on the exact property; the name Mill Hill and an NW7 postcode answer none of them. Use the government’s council lookup first; after that, work through the conservation area boundary map, the listed building records, the Article 4 orders and the January 1991 schedule of properties, the title or lease and the planning history of the property. The 2008 appraisal proposed taking one Marsh Lane house out of the boundary, so rely on the current map rather than older plans.

Local design and consent context

Planning data records the Mill Hill Conservation Area, reference CA13, as a London Borough of Barnet designation dating from December 1968, and Barnet’s Character Appraisal Statement is marked adopted in April 2008. The appraisal divides the area into six character areas: Nan Clarks Lane and Highwood Hill; The Forge and Lawrence Street; The Ridgeway; the Three Hammers T-junction; Mill Hill Village; and Wills Grove and Winterstoke Gardens, with the planned development of Hammers Lane.

On its open data portal, Barnet presents its conservation area appraisals and boundary maps as the authoritative source. The published Mill Hill boundary map, dated November 2023, gives a last amended date of 26 September 1988. The appraisal describes the area as it was in 2008, when Barnet consulted a Mill Hill Conservation Area Advisory Committee on applications affecting it.

According to the 2008 appraisal, an Article 4 direction made shortly after designation covers most of the conservation area. In the parts it names, around the Old Forge, The Ridgeway, Milespit Hill, Wills Grove and Winterstoke Gardens, it lists planning controls including replacement windows and doors, roof lights, new wall materials and painting, small additions such as front porches and outbuildings, replacing boundaries other than hedges, hardstandings in front gardens, vehicle crossovers, and most new gates, fences and walls.

Access and site planning

Delivery stops, waste storage and protection for any shared hall, stair or drive all need a plan before work starts. Barnet’s 2008 appraisal describes The Ridgeway and Highwood Hill as busy vehicular routes, with heavy school traffic on The Ridgeway, Wills Grove as a private shared-surface road with patchy, privately maintained surfacing, and Wills Grove and Winterstoke Gardens as informal roads with no pedestrian paths. Before scaffolding, a skip or materials go on the road or pavement, ask the highway authority which licence is needed and how much notice it requires; TfL issues such licences only on its red routes, so check whether the road outside is one.

Continue planning

The Mill Hill area guide · House extension across London · Hampstead Architecture & Planning · Hampstead Chartered Surveyors

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