Why Trees Create Complications for Building Projects
Building near trees raises three distinct sets of issues: planning protection, structural risk to the building from roots, and construction methodology to avoid damaging the tree. Each requires different professional input and needs to be addressed before construction starts, not discovered on site.
Planning Protection for Trees
In London, trees may be protected in several ways. Trees in conservation areas are automatically subject to restrictions — you must give six weeks' notice to the local authority before carrying out any works to a tree above a certain size (five centimetres in diameter at 1.5 metres from ground). The local authority can then either object (typically by serving a tree preservation order) or raise no objection within the six-week period.
Trees outside conservation areas may be covered by a tree preservation order (TPO) if the local authority has specifically designated them as being of amenity value. Working on a protected tree — including removing it, pruning it significantly, or damaging its roots — without consent is a criminal offence with potentially significant fines.
When submitting a planning application for a development that involves works near trees, the local authority will typically require an arboricultural impact assessment (AIA) and a tree survey, and may impose conditions requiring a method statement for how the construction will protect nearby trees. These surveys are carried out by a qualified arboriculturalist.
Root Protection Areas
A root protection area (RPA) is a defined zone around a tree within which construction activity is restricted to avoid damaging the tree's root system. The standard approach for calculating an RPA is based on the trunk diameter, and the zone is typically circular. Within the RPA, digging, compaction of soil, storage of materials, and driving plant or vehicles are all restricted or prohibited without specific precautions.
Extensions and structures built within the RPA of a significant tree require careful design and methodology to avoid harming the tree. Suspended slab foundations, hand-dug piled foundations, or other no-dig construction approaches may be used to allow construction without the conventional mass excavation that would damage roots.
Structural Risk: Tree Roots and Foundations
Clay soils, which are common across much of London, are susceptible to shrinkage and heave caused by tree root activity. During dry periods, tree roots extract moisture from clay, causing the soil to shrink and potentially causing subsidence of nearby structures. In wet periods, or after a tree is removed, the clay can heave as moisture is reabsorbed.
For new extensions on sites with clay soil and significant nearby trees, the depth and design of foundations must account for root influence. This is assessed by a structural engineer, who will consider the tree species (some are more aggressive than others), the height and canopy spread, the distance to the proposed structure, and the soil type. Foundation depths of three metres or more are not unusual where large trees on clay are involved.
Tree removal near existing buildings on clay can also cause damage — the heave of soil after root moisture extraction ceases can be slow and persistent. This is a consideration both for proposed construction and for managing existing trees adjacent to existing buildings.
Construction Methodology Near Trees
Where construction must take place within or near the RPA of a protected or significant tree, a construction method statement should be prepared by an arboriculturalist. This sets out exactly how the works will be carried out to minimise root damage — typically including protective fencing around the RPA, restrictions on machinery, hand-digging in sensitive zones, and monitoring during construction.
Local planning authorities will usually require this method statement as a planning condition, and compliance must be demonstrated during the works.
Getting the Right Advice Early
Tree-related issues are best identified and addressed at the pre-design stage, before drawings are finalised. At RCB Design and Build, we identify tree constraints as part of our pre-construction review and coordinate with arboriculturalists and structural engineers where tree proximity affects the design or foundation specification. Getting this right at the outset avoids expensive redesigns later. Contact us to discuss your project.