How Close Can You Build to a Tree?

Written by Tom Thompson BSc Arb MSc eFor MArborA · Arbor Cultural Ltd

Homeowners often ask for the minimum distance between a proposed extension and a tree. There is no single distance that works for every species, stem size, soil type, foundation system and site.

The planning and engineering questions also differ. BS5837 is concerned with the effect of development on trees and their future relationship with occupants, while foundation design may separately need to consider shrinkable clay and vegetation.

Start with the Root Protection Area

BS5837 uses stem diameter to calculate a Root Protection Area, or RPA. It is a defined area intended to protect enough rooting environment for a retained tree. A small young tree may generate a modest RPA; a mature tree can create a much larger design constraint.

The RPA is an important starting point, but it is not the only one. Crown spread, future growth, shade, branches over roofs, access for maintenance and the likely expectations of future occupants all influence whether a layout is sustainable.

Remember that roots do not grow as perfect circles

The familiar circular RPA on a plan is a planning tool rather than an x-ray of the root system. Real roots respond to soil conditions, old foundations, roads, water availability, compaction and other obstacles.

Where a design decision genuinely depends on actual root position, targeted Air Spade root investigation can sometimes provide direct evidence without mechanically cutting through the soil.

Planning distance and foundation depth are separate

A proposal can be acceptable from an arboricultural planning perspective and still require a particular foundation design because of shrinkable clay. Conversely, a deep or piled foundation does not automatically make an otherwise harmful planning relationship acceptable.

Where the concern is foundation design near vegetation, Arbor Cultural’s Building Control Tree Surveys service supplies the tree information needed for the Building Control or engineering question.

Can you build inside an RPA?

Sometimes limited encroachment can be considered, but it should not be treated as routine. The answer depends on tree quality, existing ground conditions, the scale and location of the incursion, and whether a construction method can genuinely protect roots and soil.

Solutions can include carefully designed no-dig surfaces, suspended structures, adjusted foundation positions or specific excavation controls. These need to be assessed in the Arboricultural Impact Assessments and, where relevant, translated into a method statement.

Early design gives you more options

If an arboriculturist first sees the scheme after the building, drainage and access are fixed, every tree issue becomes a late constraint. A survey and constraints plan during feasibility let the architect test alternatives while changes are still inexpensive.

The best outcome is usually not “maximum possible distance” but a design in which the tree has sufficient soil and space and the building works well for the people who will use it.

Practical Next Step

If you are planning an extension or new building near trees, send the current drawings before foundations and drainage are finalised. Early arboricultural input usually gives the design team more workable options.

For project-specific advice, you can contact Arbor Cultural or send your drawings and requirements through our online quotation form.