House
extensions
in Marlow, SL7
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Three lines on the map shape an extension in Marlow: the Thames flood zones, the Green Belt round the town and the Chilterns boundary along its northern edge. We place your house against all three before sketching a room.
The Local Plan is frank about the town's limits. It says the AONB, Green Belt and flood constraints mean Marlow has considerably less development than would otherwise be envisaged for a town of its size, and that development will be focused within the town and be more limited in scale. For a household wanting more room, that is a reason to test the plot before the design.
The houses being extended are varied. In the Marlow ward at the 2021 Census, 23.1 per cent of households lived in semi-detached houses and 23.4 per cent in terraces, and 32.8 per cent had three bedrooms while 21.8 per cent had two.
Sources: Wycombe District Council: Wycombe District Local Plan (adopted August 2019); ONS/Nomis: Census 2021 TS044 Accommodation type, 2022 ward Marlow (E05013153); ONS/Nomis: Census 2021 TS050 Number of bedrooms, 2022 ward Marlow (E05013153)
Extending a house near the Thames
The district's 2014 flood risk assessment starts from a presumption against all building extensions in Flood Zone 3b, and in Zones 3a and 2 it says extensions, outbuildings included, should be discouraged to avoid raising flood levels elsewhere. Around Pound Lane, in Zone 3a, and in the housing enclosed by the A404, Lock Road and the railway, that is where any plan begins.
Where an extension is acceptable, the assessment expects a flood risk assessment detailing the resilience and resistance measures in the design, floor levels generally at least 300mm above the 1 in 100 river flood level including climate change, and dry access for residential property in that event.
Size alone does not settle it. The assessment notes that minor developments under permitted development rights, some extensions among them, can raise flood risk where they affect a floodplain or flood flows.
Away from the river zones, two significant critical drainage areas still drain through Marlow to the Thames. Inside those, extensions and outbuildings can obstruct overland flow paths, and the assessment asks for them to be designed so they do not raise the flood risk to adjoining properties.
Sources: Wycombe District Council: Strategic Flood Risk Assessment Level 1 Update (November 2014)
The edges of the town: Green Belt volume and the Chilterns
The Local Plan calls Marlow severely constrained by Green Belt and protects the Green Belt shown on its Policies Map. For a house inside it, policy DM43 accepts an extension only when one or more of its conditions is met, measured as gross external volume including any basement; the full test is on our Wycombe page.
In its Regulation 10A review of June 2025 the council rates DM42 and DM43 as partially out of date. National policy treats an extension as not inappropriate where it does not add a disproportionate increase over the original building, which means the building as it stood on 1 July 1948, or as first built if later.
On the northern fringe the Chilterns National Landscape comes in. A point on Marlow's northern edge lies inside it and the town-centre point outside, and on such article 2(3) land permitted development covers neither a side extension nor a rear extension of more than one storey.
Everywhere in the town, a single-storey rear extension under permitted development may not project past the original rear wall by over 4 metres on a detached house, or by over 3 metres on any other.
Sources: Wycombe District Council: Wycombe District Local Plan (adopted August 2019); Buckinghamshire Council: Regulation 10A Review of the Wycombe District Local Plan and the Delivery and Site Allocations Plan (June 2025); MHCLG (GOV.UK): NPPF Chapter 13, Protecting Green Belt land (GB1-8) (August 2026); planning.data.gov.uk (MHCLG, Natural England data): AONB point query 51.585,-0.785 (north edge of Marlow); planning.data.gov.uk (MHCLG, Natural England data): AONB point query 51.5707,-0.7756 (Marlow centre); legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 1 Part 1, Article 2(3) land (latest available revised); legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 2 Part 1, development within the curtilage of a dwellinghouse (latest available revised)
Neighbours, the town council and building control
Applications go to Buckinghamshire Council, which asks neighbouring properties and Marlow Town Council for their views. Among the issues the town council can raise are loss of privacy, overshadowing or an overbearing effect, and design and materials, so we show those relationships on the drawings.
Building close to or on a shared boundary means notifying the neighbour under the party wall rules, which sit apart from both planning permission and building regulations approval. Foundations dug within 3 metres of a neighbour's building and deeper than its footings are notifiable too.
The Building Regulations 2010 cover the extension of buildings. Approval comes from Buckinghamshire Council's building control service, or from a private registered building control approver if you would rather appoint one.
Our London team designs and builds Marlow extensions from the Finchley Road studio in Hampstead: in-house architects and designers for the plan and elevations, our own structural engineers for footings and steel, and Hampstead Chartered Surveyors & Building Consultancy for the measured survey of the existing house.
Outside Marlow, and in London rather than Buckinghamshire: Project example: Hampstead Garden Suburb, NW11. That project of ours added a rear extension of a single storey to an Arts and Crafts house, and had to win consent from both the Hampstead Garden Suburb Trust and the council, as its case study records.
Sources: Marlow Town Council: Planning & Transport Committee; GOV.UK: Party walls and building work - overview; GOV.UK: Party walls and building work (printable guide); GOV.UK: The Party Wall etc. Act 1996 explanatory booklet; GOV.UK: Building regulations approval - when you need approval; Buckinghamshire Council: Building regulations and control; GOV.UK: Building regulations approval (printable guide); Hampstead Renovations: Meadway, NW11 case study
House Extensions in Marlow: Questions
Is my Marlow house in the Green Belt, and what is the council's test for an extension?
That depends on the address: the Local Plan protects the Green Belt shown on its Policies Map and describes Marlow as severely constrained by it. For a house inside it, policy DM43 accepts an extension only when one or more of its conditions is met, using gross external volume including any basement, compared with the original building. The council's June 2025 assessment rates DM43 partially out of date, so national Green Belt policy is weighed too.
Does an extension in the Chilterns National Landscape need planning permission?
More often than elsewhere. Areas of outstanding natural beauty, the Chilterns among them, are article 2(3) land, where permitted development covers no side extension, no rear extension of more than one storey and no cladding in stone, render, timber, plastic or tiles. A point on Marlow's northern edge lies inside the Chilterns while the centre does not, so we check your address before design.
How does flood risk affect an extension in Marlow?
Heavily near the river. The 2014 assessment starts from a presumption against building extensions in Flood Zone 3b, and discourages them in Zones 3a and 2 so as not to raise flood levels elsewhere. Where one is acceptable, a flood risk assessment sets out resilience measures and floor levels, generally 300mm above the 1 in 100 river flood level allowing for climate change. The Environment Agency's flood map for planning shows your zone.
Who signs off building regulations for an extension in Marlow?
You have a choice between Buckinghamshire Council's building control service and a private registered building control approver; an approver informs the council through an initial notice. Building regulations approval is a separate matter from planning permission, and an extension usually needs both. We submit the structural calculations and drawings to whichever body you pick and arrange its inspections.
Do I need to tell my neighbours before extending a Marlow house?
Under the party wall rules, yes, if the work is on or near a shared boundary or the foundations go within 3 metres of their building and below its footings. Separately, when an application is made, the council asks neighbouring properties for their views, as it asks Marlow Town Council.
How should I prepare my Marlow home for a feasibility visit about an extension?
Have the title plan to hand if you can, with any earlier planning decisions and completion certificates for previous additions, since those affect what can still be built. Clear a path to the side and rear walls and to any manhole covers, and tell us about any flooding the garden has had. We bring the flood zone and Green Belt checks for the address with us.
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