Shrubs and Dense Planting Close to Foundations in Clay

Most clients are aware that mature trees near foundations on clay soil can cause problems. Fewer realise that large shrubs can be just as significant — and that some common garden plants trigger the same foundation depth requirements as trees.

Which Plants Are Treated as Trees?

Under NHBC Standards Clause D6(e), any shrub that has grown to a height of 1.8 metres or greater must be treated as a tree for the purpose of foundation depth assessment. Species that commonly reach this height and are frequently found in UK gardens include:

– Forsythia — often left to grow unchecked to well over 2 metres
– Choisya (Mexican orange blossom)
– Ceanothus
– Buddleja (butterfly bush)
– Large ornamental grasses in some classifications

If these plants are growing on clay within 20 metres of your proposed foundation, their influence on foundation depth must be assessed using the same approach as for trees.

What Foundation Depth Is Required?

Where a shrub classified as a tree is within its calculated zone of influence on a clay site, the minimum foundation specification is:

– Minimum foundation depth: 1.5 metres below ground level
– Expanded polystyrene heave protection beneath and around the foundation
– Suspended ground floor with a 300 mm void beneath the slab
– If classified as a tree (over 1.8 m height): minimum 2.2 metres depth may apply

What About Removal of the Shrub?

Removing the shrub does not immediately recover the situation. Clay takes 2–3 years to recover after the root system has been eliminated — the moisture state of the ground remains affected during that period because the plant was extracting moisture and drying the clay. A removed shrub is not treated as absent for foundation design purposes until that recovery period has elapsed. The full foundation specification must be maintained regardless.

Species That Are Not Influential

Not every plant triggers these requirements. A small ornamental tree or Japanese Maple more than 7 metres away, or a conifer more than 10 metres away, may fall outside its zone of influence and require only standard minimum foundation depths.

The LABC Foundation Depth Calculator can be used to check whether specific trees and shrubs are within their influence zone for your site.

Need Your Foundation Checked?

If you have mature shrubs or planting near your proposed extension on a clay site, PorthouseDean can advise on the foundation specification needed. Get a quote to discuss your project.