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CASE STUDY · APPROVED

Garage conversion and rear conservatory for a ground-floor flat

Marsh Lane, Mill Hill · London Borough of Barnet

Conversion of a detached garage into a habitable room, a single-storey rear conservatory, alterations to the rear roof with three rooflights and new gates for a ground-floor flat in a detached building of three flats. We prepared the full planning application with a design and access statement, fire safety statement and drawings, followed by an amended conservatory scheme with a tree protection scheme and biodiversity net gain exemption statement.

Rear of the building before works (Design and Access Statement)
Rear of the building before works (Design and Access Statement)

The site

The site is a three-storey detached red-brick building on a residential street in Mill Hill, divided into three self-contained flats, with the application flat on the ground floor and a detached garage to the rear. The building is not in a conservation area and is not statutorily or locally listed, but the property falls under Article 4(1) directions, lies within an SSSI impact risk zone and is covered by a Tree Preservation Order. The site has a PTAL of 1a.

The challenge

The extensions had to remain subordinate to the host building and respect its form and materials under Barnet policy DM01 and the Residential Design Guidance SPD, without harming neighbouring amenity. Protected trees in the rear garden needed safeguarding during construction, and a pre-commencement tree protection condition applied to the first permission.

What was submitted

  1. Design and Access StatementAssessed the scheme against the London Plan, Barnet policies DM01, DM02 and DM17 and the Residential Design Guidance SPD. The conservatory extends the footprint 5m to the rear and 2.6m towards the side boundary with a glass roof, the garage becomes a gym with external insulation and render, and the flat goes from three to four bedrooms, with the three existing parking spaces retained.
  2. Fire Safety StatementAddressed London Plan Policy D12(A), including a fire detection and alarm system to BS 5839-6, 30-minute fire resistance to structural elements, emergency egress windows and fire appliance access from the street within 45m of the furthest point of the flat.
  3. Existing and proposed plans, elevations and sectionsDrawings show the conservatory with a glass roof, uPVC doors and windows matched to the existing, exposed brick walls to match, and alterations to the existing hip roof in matching colours and materials.
  4. Tree Protection Scheme and PlanPrepared to BS 5837:2012 to meet the tree protection condition, retaining a fastigiate yew and a Lomatia in the rear garden with protection measures, and removing five garden shrubs and climbers.
  5. Biodiversity Net Gain Statement / ExemptionSet out that the amended scheme is exempt from statutory biodiversity net gain, as the conservatory has an area of 13.2 square metres and its roof does not exceed the eaves height of the existing kitchen roof.

Outcome

The full application was approved by the London Borough of Barnet on 4 December 2024. An amended single-storey rear conservatory and rear roof alteration scheme (25/0105/FUL) was approved on 10 March 2025.

From the reports

Proposed rear elevation showing the glass-roofed conservatory (existing and proposed rear elevation)
Proposed rear elevation showing the glass-roofed conservatory (existing and proposed rear elevation)
Proposed section through the conservatory and altered hip roof (existing and proposed section A-A)
Proposed section through the conservatory and altered hip roof (existing and proposed section A-A)

Images and findings are taken from the documents submitted with the planning application. The full application is available on the council's planning register.

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