Building Near Trees in London: Root Protection, TPOs and Planning

Before any works affecting trees on or near a London property, the tree's protection status must be confirmed.

• **Tree Preservation Orders (TPOs):**
• A TPO is a formal protection order made by the local planning authority on a specific tree or group of trees. Once a TPO is in place:
• •Cutting down, uprooting, topping, lopping, or wilfully damaging a TPO-protected tree without consent is a criminal offence
• •Prior written consent from the local planning authority is required for any works to a TPO-protected tree
• •Application for TPO works is free and must be decided within 8 weeks

**Conservation Area tree notification:** In a Conservation Area, any tree with a trunk diameter of 75mm or more at 1.5m above ground has de facto protection — you must give the local planning authority 6 weeks' prior written notice of any proposed works to such a tree. The council can make a TPO during this 6-week period.

**Checking TPO status:** Most London boroughs have an interactive map showing TPO locations. Check the borough's planning portal before planning any works that might affect trees on or adjacent to your property. The borough's tree officer can advise on the status of specific trees.

**Building near trees — the tree's rights:** Even where a tree is not TPO-protected or in a Conservation Area, the roots of a tree may extend significantly beyond the canopy drip-line — root protection zones (the area that must be preserved to maintain the tree's health) can extend to distances of 12 × the trunk diameter from the base of the tree for some species. Damaging root systems can kill or destabilise a tree, potentially creating a neighbour dispute and a legal liability.

The combination of London clay soil and the presence of trees is the primary foundation design challenge in London residential construction.

**London clay and clay shrink-swell:** London clay is an expansive soil — it shrinks in dry conditions (when trees absorb water during the growing season) and swells in wet conditions (when trees are not actively absorbing water, or after removal). This seasonal movement can cause foundation movement, cracking, and subsidence where foundations are not designed to account for it.

• **The tree-foundation interaction:**
• •During dry summers, tree roots draw moisture from the soil under and around the foundation — clay shrinks, and the foundation settles
• •If a tree is removed, the former moisture demand drops — clay swells, and foundations can heave upward (sometimes more damaging than the original shrinkage)
• •The zone of influence of a tree depends on the species, the trunk size, and the soil moisture conditions

**Calculating the influence zone:** The National House Building Council (NHBC) and Building Research Establishment (BRE) provide guidance tables (the 'BRE tables') that classify tree species by water demand and calculate the influence zone as a function of trunk size and mature height. For high-water-demand species (oak, elm, willow, poplar) near London clay, influence zones can extend 20–30m or more.

• **Foundation design for building near trees:**
• Where an extension is being built within the influence zone of a tree (or a recently removed tree), the foundation must be designed to account for clay shrink-swell movement:
• •Standard strip foundations are typically not appropriate within the influence zone
• •Mini-piled foundations (piles driven to depths below the zone of clay movement — typically 10–15m in London) are the standard solution
• •The pile depth is calculated by the structural engineer based on the BRE tables and the specific site conditions
• •Mini-pile installation for a typical London rear extension in a tree-proximity situation: £15,000–£35,000 (a significant but unavoidable cost where tree influence is confirmed)

**Tree removal and heave:** Removing a tree to allow construction is not a straightforward solution. The clay will swell after removal (heave), and this process can take years to stabilise. A structural engineer must assess whether removal improves or worsens the foundation design requirement for a specific project.

For any construction project in proximity to significant trees, two specialist advisors are key:

• **Arboriculturalist:**
• A qualified arborist (AA, MArborA) can:
• •Assess the tree's health, species, root system, and proximity to the proposed works
• •Carry out a BS 5837:2012 'Trees in relation to design, demolition and construction' survey and report
• •Assess the root protection area (RPA) — the circular zone around the trunk calculated from trunk diameter
• •Advise on tree protection measures during construction (fencing, no-dig zones, root barriers)
• •Prepare the arboricultural impact assessment required for planning applications affecting trees

• **The BS 5837 survey:**
• A BS 5837 arboricultural survey is the standard required by planning authorities when trees are present on or near a development site. It:
• •Categorises trees by quality and retention value (Category A, B, C)
• •Defines root protection areas for each tree
• •Specifies the tree constraints plan (TPC) — the area excluded from development and site operations
• •Informs the design of the extension (setback distance from trees, no-dig foundation requirements)

**Structural engineer:** The structural engineer uses the arboricultural report (specifically the tree species, trunk size, and proximity to the works) along with the site soil survey to design foundations appropriate for the tree-clay interaction.

**Planning implications of trees:** Where a planning application involves works near trees, an arboricultural impact assessment and tree constraints plan are typically required as part of the application documents. The planning officer will consult the borough's tree officer, who will assess the impact on tree health and survival and may set planning conditions requiring tree protection measures during construction.