House Extensions Mill Hill

GS Renovation extends houses across Mill Hill, where the plots are generous, the ground is largely London Clay and the planning position changes street by street. Rear and kitchen extensions, side return, wrap around and double storey, all built to the structural engineer's design and inspected by Barnet Council's building control, with our founder's 30+ years of hands-on experience behind the work.

In Mill Hill, a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m² (indicative London ranges, Architecture for London, Resi, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Mill Hill: the structural engineer we appoint designs the structure, we build it, and building control signs it off.

Foundations on Mill Hill's clay

Extension foundations here start with a hole, not a price. We dig trial holes to find what is actually under your garden, because the ridge does not sit on one uniform stratum and the depth your house was founded at in 1935 tells you very little about what the new work needs today. On shrinkable clay the foundation has to go below the zone that dries out seasonally, and nearby trees push it deeper again, following the engineer's design and the standard tables relating species, mature height and distance from the trench.

Where trees are close, depth on its own is not the whole answer. Clay dried out by roots will swell again if those roots are cut or the tree dies back, so the design often includes compressible board against the face of the foundation and a void former beneath any ground bearing slab, giving the clay somewhere to go instead of lifting your new floor. Where a tree is protected, or stands within the conservation area, the engineer designs around it and you notify Barnet rather than quietly taking it down.

Housing in Mill Hill

Mill Hill counts roughly 10,980 homes for Council Tax; 10% are older than 1919, 38% interwar and 52% post-war, with 44% flats and 56% houses. 24% of homes here are terraced; a rear extension on a terrace in Mill Hill digs foundations close to both neighbours, so notices to both are normal.

Community Infrastructure Levy and a large Mill Hill extension

Barnet Council was one of the first London authorities to adopt and implement the Community Infrastructure Levy, in May 2013, and its revised charging schedule came into effect on 1 April 2022. The levy is charged on new residential floorspace, and a large householder extension on one of the semi-detached or detached houses that make up 29% of Mill Hill's homes can fall within it. Extensions to your own home can be exempt, but the exemption is not automatic. It has to be claimed before work starts, and the start of work has to be notified to the council on the prescribed form.

The trap is the sequence. If the groundworks begin before the exemption has been claimed, it cannot be claimed afterwards, and a missing commencement notice can bring a surcharge on top. On a Mill Hill extension the CIL forms therefore sit in the programme alongside the planning decision and the party wall notices, and the first trench is not dug until the council has acknowledged them. Where the house is inside the Mill Hill Conservation Area, which covers approximately 152 hectares, the planning route is also narrower, so the paperwork timetable starts earlier still.

Green Belt, conservation area and permitted development

Mill Hill's planning position is unusually varied for one postcode. Land to the north and east falls within the Green Belt, where an extension is judged on whether it is a disproportionate addition over and above the size of the original building, a test that bites well before the usual householder limits do. Mill Hill Village and The Ridgeway sit inside a conservation area, where permitted development rights are narrower and can be cut back further by an Article 4 direction. The interwar streets near the Broadway are usually the most straightforward.

So we establish the route before anything is drawn: permitted development, the larger householder extension prior approval procedure, or a full householder application to Barnet Council. The Building Regulations apply either way, covering structure, insulation, drainage and fire safety. On the excavation side, digging foundations within three metres of a neighbour's building and below their foundation level is notifiable under the Party Wall etc. Act 1996 even between two detached houses. We give you the drawings and the timing the notice needs; you serve it as building owner, or your surveyor does, and we build to any award.

Kitchen extensions in Mill Hill

52% of the houses in Mill Hill are semi-detached or detached, which usually leaves room for a kitchen extension across the full rear width, with a steel beam over the opening into the house. The fit-out is priced on its own page, kitchen renovation in Mill Hill; the build figures are in the table above.

Side return extensions in Mill Hill

Side returns belong to Victorian and Edwardian terraces, and only 10% of homes in Mill Hill predate 1919, so they are the exception here; most extensions run across the full rear width instead. Our side return extension page covers the build in detail, with build rates of £2,400 to £3,200 per m².

Double storey extensions in Mill Hill

Under permitted development a two-storey rear extension can go no more than 3 metres beyond the original rear wall and must stay at least 7 metres from the rear boundary, and in a conservation area it is not permitted development at all. Many gardens are too short for the 7 metre rule, so a double storey extension in Mill Hill often needs planning permission from Barnet Council. With 29% of homes here semi-detached or detached, the side of the house is often an option too. The Mill Hill Conservation Area covers approximately 152 hectares of land towards the northwest corner of the borough (council source). Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.

• Levels, drainage and landscaping made good around the new build
• Trial holes to set foundation depth near drains and trees
• Flat and pitched roofs with rooflights or lanterns

VOA Council Tax stock of properties, March 2025, for Mill Hill Broadway, Mill Hill East and Mill Hill Park. Counts rounded by the VOA.

Related reading for Mill Hill homeowners planning house extensions: RSJ and steel beam installation in Mill Hill, house extensions across Barnet, wraparound extensions, garage conversions in London, our London house extensions page and Basement Conversion Cost.

The GS Renovation Difference in Mill Hill

Extensions in NW7 go wrong at two ends, the ground and the consent. We deal with both first, with trial holes and an engineer's foundation design at one end and an honest read of the Green Belt, conservation area and permitted development position at the other. Then one team builds it through to decoration, inspected by Barnet Council's building control at each stage as the work proceeds.

• Brick and mortar matched to the existing elevation
• Paperwork and certificates handed over for when you sell
• Building control inspected at foundation, damp course and steel stages

Checked by Abdelaaziz Saaid, founder and director of GS Renovation Ltd (Companies House no. 14611254). Structural design is by the consulting engineer appointed to your project.