How Close Can You Build to a 3-Metre Tall Magnolia Tree?
When planning a new extension, garden room, or even a small outbuilding, existing trees on your property can present both a design opportunity and a practical challenge. One common question we encounter is:
“How close can I build to a magnolia tree?”
In this post, we’ll walk you through what to consider when building near a 3-metre tall magnolia with a 150mm trunk, and why keeping a respectful distance is vital for both the health of the tree and the safety of your structure.
Understanding Magnolia Trees
Magnolias are popular ornamental trees known for their beautiful blooms and elegant structure. While they aren’t known for aggressive root systems like willows or poplars, they still have wide, shallow roots that need space to absorb water and nutrients.
Even a small magnolia tree relies on an underground network of fine feeder roots, which can be easily damaged by excavation or construction.
Step 1: Apply the Root Protection Area (RPA) Formula
In the UK, tree protection during construction is guided by BS 5837 – the British Standard for trees in relation to design, demolition, and construction.
This standard uses a simple formula to calculate the Root Protection Area (RPA) – the minimum area around a tree that should remain undisturbed:
RPA radius = 12 × stem diameter (in metres)
For your magnolia:
Trunk diameter = 150 mm (0.15 m)
RPA = 12 × 0.15 = 1.8 metres
➡️ You should ideally build no closer than 1.8 metres from the trunk.
This radius forms a circular zone around the tree where soil, roots, and moisture conditions should be protected.
Step 2: Consider Site Conditions and Foundation Types
If you’re tight on space and considering building closer than the recommended 1.8 metres, it’s possible—but only with extra precautions:
✔️ Possible Solutions:
Use low-impact foundations, like pile and beam or mini-piles, which can bridge over root zones.
Elevate the structure (e.g., raised decking or a floating slab) to avoid root damage.
Engage a qualified arborist to assess root locations and recommend root pruning if absolutely necessary.
❌ Avoid:
Strip foundations or trench digging close to the tree.
Heavy machinery or soil compaction within the RPA.
Raising ground levels, which can suffocate roots.
Step 3: Think Long-Term
Building too close to a tree, even a small one, can lead to:
Root damage and tree stress, making the tree more susceptible to disease or dieback.
Soil movement in clay-rich soils due to moisture changes, which can cause subsidence or heave.
Future maintenance issues, especially if the tree grows and its roots expand.
Also, if the tree is protected by a Tree Preservation Order (TPO) or located in a Conservation Area, you'll need permission from your local planning authority for any work that affects it.
Conclusion
For a 3-metre tall magnolia tree with a 150mm trunk, you should aim to keep your construction at least 1.8 metres away. While this may seem like a small buffer, it can make a big difference in maintaining a healthy tree and avoiding costly problems down the line.
If you’re planning a build near a tree, it’s always wise to consult both an architect and a tree professional to strike the right balance between design and preservation.