Three numbers, a design rule
The 3-30-300 rule fits in one sentence: from every home you should see at least three trees. The neighbourhood should have at least thirty per cent tree canopy cover. A public green space should be no more than three hundred metres away. It was set out in 2021 by the urban forester Cecil Konijnendijk and has since been adopted as a planning benchmark by several European cities.
It is worth saying straight away what it is. This is not a threshold derived from a dose-response curve, the way 53 decibels is for road noise. It is a deliberately memorable synthesis, assembled from a body of work of uneven quality, to give local authorities a target that can be checked on a map. That difference in status does not disqualify it: it tells you how to read it. A home that meets the rule is not thereby a healthy home, and a home that fails it is not a home to avoid.
Each number answers a different question. The three trees concern daily visual exposure, from inside, where people spend most of their time. The thirty per cent concerns the neighbourhood as a physical system: shade, surface temperature, rainwater. The three hundred metres concern use, since the share of residents who actually go to a green space falls sharply with walking distance. Read that way, the rule is less a health claim than a checklist covering three scales at once, which is precisely what most property assessments fail to do.
Ulrich, 1984, and what gets claimed for it
The study everyone cites is Roger Ulrich’s, published in Science in 1984. It compared recovery after gallbladder surgery in a Pennsylvania hospital between 1972 and 1981, depending on whether the room looked onto a stand of trees or onto a brick wall. Patients on the tree side went home slightly earlier, roughly eight days against eight and a half, received fewer strong analgesics, and drew fewer negative comments in nursing notes.
The finding deserves to be reported with its limits, which are real. The sample was twenty-three matched pairs. It was one hospital, one procedure, a homogeneous population, and room allocation was not randomised in the sense a clinical trial would require. The paper has been cited enormously and replicated with better methods rather rarely. It opened a research field, which is already a great deal. On its own it does not show that a view of trees affects the health of a healthy occupant, at home, over years.
What holds up, what is thinner
The best established part of the rule is physical rather than psychological. Dense canopy measurably lowers air temperature and, above all, felt temperature in summer: several degrees between a planted street and a bare one, more at surface level. You can verify it with a thermometer, it rests on no assumption about human behaviour, and it bears directly on heat episodes.
Reasonably well established too: proximity to accessible green space goes with more frequent physical activity, and laboratory work on attention restoration after exposure to natural settings gives consistent results, albeit over short durations and artificial tasks.
Thinner: the causal chain from neighbourhood greenery to long-term health. Observational studies find associations, but leafy neighbourhoods are also wealthier, quieter and less exposed to traffic. The share attributable to the trees themselves is hard to isolate.
One point calls for caution. Fine particles are the leading environmental risk factor worldwide, and it is tempting to present trees as a filter. The real effect on concentrations is modest, and in a narrow street lined with tall buildings a continuous tree canopy can actually restrict dispersion and raise concentrations at pedestrian level. Planting is useful for heat, marginally for noise, and for comfort. It is not an air quality policy.
Checking the rule from an address
The virtue of this rule is that it can be checked in half an hour, with no equipment.
| Criterion | How to check | The trap |
|---|---|---|
| 3 visible trees | From every habitable room window, not the shared balcony | Counting trees standing on a developable plot |
| 30 % canopy | Recent aerial imagery, canopy layers published by the city | Confusing lawn with tree cover: only canopy counts |
| Park within 300 m | Actual walking route, not straight-line distance | A park locked at night, or cut off by an impassable road |
Three further checks separate a viewing from an assessment. Look at aerial imagery from ten years ago to see the trend, since canopy can be in retreat. Check in winter, because deciduous trees screen very little from November to March and the June view is not the January view. And look up planning applications nearby, because today’s open outlook can be a building site in eighteen months.
A flat with no view
Many homes will never meet the first criterion, and it would be dishonest to claim this can be made up for indoors.
What has a serious basis concerns light and viewing distance: a room that receives direct daylight in the morning, an unobstructed window that lets the eye focus far away, a desk facing that window rather than a wall. A daily trip outside, however brief, also delivers light levels that no interior comes close to.
What has a thin basis: images of nature on the wall, tested mostly in laboratories on short-lived effects. What has no basis at all: the idea that a few houseplants clean the air of a home. The experiments cited in support were run in small sealed chambers. Scaled to a real room, they would require a number of plants no one could fit. Air renewal does that work, not foliage. Keep a plant because it is pleasant, which is reason enough.
Lif8 assesses places to live, including exposure to vegetation and daylight, and provides no form of care. None of this replaces medical advice.
