Extensions in Conservation Areas: What's Allowed?
Living in a conservation area does not mean you cannot extend — but the rules are stricter. A practical guide to what is allowed and how to get planning approval in Dorset's conservation areas.
How conservation areas affect Permitted Development
Conservation areas restrict but do not eliminate Permitted Development rights. The key restrictions are: no side extensions under PD (you need planning permission for any side extension), no rear extensions of more than one storey under PD, no cladding to the exterior, and no roof extensions (including dormers) that face a highway. Single-storey rear extensions are still permitted under PD, subject to the usual depth limits (3m for terraced and semi-detached, 4m for detached — or up to 6m/8m via Prior Approval). In Dorset, conservation areas include central Christchurch, Wimborne Minster, Wareham, Swanage town centre, Corfe Castle, much of Poole Old Town, and Westbourne in Bournemouth. Each has its own character appraisal document that sets out what the council considers important.
Getting planning permission in a conservation area
When you need planning permission (for side extensions, two-storey extensions, or anything exceeding PD), the council assesses the impact on the character and appearance of the conservation area. This is a higher bar than a standard planning application. You will need to demonstrate that your extension: preserves or enhances the character of the area, uses appropriate materials, respects the scale and proportions of neighbouring buildings, and does not harm important views or the setting of nearby listed buildings. A well-written Design and Access Statement is essential — explain your design choices and how they respond to the local context. Pre-application advice from Dorset Council costs £150–£300 but is worth it — the planning officer will tell you what is likely to be approved before you spend money on a full application.
Materials and design in conservation areas
Material choice is where most conservation area applications succeed or fail. uPVC windows are almost always refused in conservation areas — timber or aluminium alternatives are expected. Roof materials should match or complement the existing: clay tiles, natural slate, or in some Dorset areas, Purbeck stone roofing. Brick should match the existing in colour, texture, and bond pattern. Render should be painted or lime-based, not bare cement. Rainwater goods should be cast iron or cast aluminium, not plastic. These requirements add 10–20% to the build cost compared to a standard extension, but there is no way around them — using the wrong materials will result in refusal, enforcement action, or a requirement to replace after construction.
Trees and demolition in conservation areas
Two additional rules catch people out. First, all trees in a conservation area with a trunk diameter over 75mm (measured at 1.5m above ground) are automatically protected — you must give six weeks' notice to the council before doing any work to them, including pruning. If a protected tree is in the way of your extension, you need the council's permission to remove it, and they will usually require a replacement. Second, demolition in a conservation area requires planning permission. This includes demolishing a wall, outbuilding, or boundary structure. If your extension involves demolishing an existing garage or outhouse to make space, apply for permission before the builders arrive.
Written by the PlanBuildCo team
9 years designing extensions and renovations in Poole, Dorset.
Ready to take the next step?
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