Trees get blamed for a great deal of damage they did not cause, and occasionally cause damage nobody noticed until a wall cracked. Both mistakes are expensive.
This checker gives you an indicative read from four things: what the species is, how tall it will get, how close it is, and whether you are on shrinkable clay. It is a prompt to look properly, not a survey — and the method is set out in full so you can see exactly what it is doing.
Why the answer is so often "reduce it, don't remove it"
On shrinkable clay, a large thirsty tree keeps the soil around it drier than it would otherwise be. Take the tree out and the soil rehydrates and swells, over several years, lifting whatever sits on it. That is heave, and repairing it frequently costs more than the subsidence people were trying to prevent.
This is why the standard answer for a mature tree close to a house on clay is a managed crown reduction on a cycle, keeping water demand roughly constant, rather than felling. A firm that proposes removal without asking about the soil or the foundations is not a firm to take advice from.
If you can already see cracks
Stop, and take a different route: tell your buildings insurer before any work is done. Most policies cover investigation, several causes look identical from the outside — drains, made-up ground, ordinary settlement, heave — and work carried out before the insurer has seen it can affect a claim.
Questions
Can I use this for a mortgage or an insurance claim?
No. This is an indicative check for homeowners. Lenders, insurers and surveyors work to the NHBC Standards, which also account for soil plasticity and foundation depth. If money depends on the answer, pay for a survey.
The tree was here before the house. Does that matter?
Legally, less than you would hope; practically, quite a lot. A house built after the tree will usually have foundations designed with it in mind. A tree planted after the house, growing towards maturity beside shallow Victorian footings, is the combination that causes trouble.
Is my neighbour liable if their tree damages my house?
Possibly, once they know or ought to know there is a risk. That is why anything you have noticed should go to them in writing, dated, with photographs.
Which trees are the thirsty ones?
Oak, poplar, willow, elm and eucalyptus are the high water demand species in this context. Ash, sycamore, lime, maple, horse chestnut and beech sit in the middle. Fruit trees, birch and hawthorn are comparatively low.
This tool is general information for homeowners in England, not legal advice. Rules differ in Scotland, Wales and Northern Ireland. Councils change their processes: always confirm with yours before work starts.