Why Trees Matter in London Planning
London has an extensive urban tree canopy that provides environmental, amenity, and biodiversity benefits. The planning system protects significant trees — and the presence of trees near a proposed development affects:
- Whether planning permission requires tree-specific conditions
- How foundations must be designed
- What structural precautions must be taken during construction
Tree Preservation Orders (TPOs)
A Tree Preservation Order (TPO) is a formal protection order made by the local planning authority. It is an offence to:
- Cut down a TPO tree
- Prune, lop, or carry out any works to a TPO tree
- Uproot or damage a TPO tree
...without prior written consent from the planning authority.
Individual trees, groups of trees, or woodlands can be subject to a TPO. TPOs typically cover significant trees in terms of size, species, visual amenity, or ecological value.
To find out if a tree is protected: Check the local authority's interactive TPO map (available on most London borough websites) or submit a written enquiry.
Trees in Conservation Areas
In a Conservation Area, all trees with a trunk circumference of 75mm or more (at 1.5m from ground level) are protected — regardless of whether they have a TPO. Six weeks' notice must be given to the local planning authority before any works to such trees. The authority may confirm the works are acceptable, impose conditions, or make a TPO to prevent the works.
Planning Applications and Trees
If your proposed extension or new structure is within or near the root protection zone of a significant tree, the planning application must be accompanied by:
- An arboricultural impact assessment (BS 5837 compliant)
- A tree survey identifying all trees on and adjacent to the site
- A root protection zone calculation for each significant tree
- Proposed protective measures during construction
The local authority's arboricultural officer will review these documents and may impose conditions on the planning permission requiring tree protection measures.
Root Protection Zones (RPZ)
The root protection zone (RPZ) is a calculated area around a tree within which the ground must not be disturbed, compacted, or excavated. It is calculated as 12 times the stem diameter (at 1.5m from ground level), forming a circle around the trunk.
For example, a tree with a 300mm trunk diameter has an RPZ radius of 3.6m.
Within the RPZ:
- No excavation for foundations
- No trenching for drainage or services
- No storage of materials or machinery
- No change to soil levels
Foundation Design Near Trees
When the extension footprint is near — but not within — the RPZ, or where the RPZ cannot be entirely avoided, the structural engineer must specify an appropriate foundation type:
Mini-Piles
Piles are driven through or past the root zone to stable ground, with a reinforced concrete ground beam spanning between pile heads. This minimises excavation and avoids disturbing the root zone.
Raft Foundations
A reinforced concrete raft spanning the root zone. The raft is designed to accept differential settlement if root activity causes localised ground movement beneath it.
Extended Trench Foundations
Standard strip foundations can sometimes be deepened to pass below the root zone in more moderate cases. Consult a structural engineer.
London Clay and Shrinkage Risk
The combination of London Clay and tree roots creates the specific shrinkage risk described in our guide on underpinning. When a large tree is removed (or dies), the clay beneath the former root zone will gradually re-wet and expand — potentially causing heave of existing foundations. This is as significant a risk as shrinkage during a dry summer.
Heave is particularly relevant when an extension is proposed next to a tree that is to be removed as part of the project.
Planning Conditions and Tree Protection
Typical planning conditions relating to trees include:
- Pre-commencement condition requiring an approved tree protection plan before work starts
- Requirement for physical tree protection barriers (as per BS 5837) erected before any machinery accesses the site
- Prohibition on any works within the RPZ during construction
- Requirement for a post-construction arboricultural monitoring report
RCB Design & Build
RCB coordinates arboricultural input and structural foundation design for projects near trees as part of the pre-construction planning process. We engage qualified arboriculturalists for BS 5837 surveys and work with structural engineers to ensure foundation design is appropriate for the site conditions.