Well yes that does happen 🙄 and more fool them for not sorting that before going ahead with extension.
but it is often the other way round, and it can often be that no one actually planted deliberately. Many houses have boundary borders against the fence. One over zealous squirrel can create a sapling chestnut that no one sees at the back of the border for some years. Same with birch seeds, and acorns. Before anyone realises there’s a young tree 6 foot high showing above the fence at the back of the border. The owners go” awww, that’s nice..I’ve got an oak tree in my garden” and don’t take it out whilst they still can….then it just gets taller and taller, till they claim it’s always been there .
whist some trees like oak and beech take a while to grow and give time, birch and chestnut shoot up and make nuisance trees in the wrong place within a few short years.
as I said I love trees, every garden needs trees for insects and birds, but it must be right tree, right place. Unless you’ve got a garden at least 30m long and wide, or boundaries run to open rural space, there isn’t room for a full size mature oak, horse chestnut or other big native species. Sure, if you inherit a tree with a preservation order than you have to leave it but the building is usually built accounting for that.
so, my take is that if it were My neighbours who wanted to extend to within 3 metres of my trees, I would negotiate with them to get it removed and replaced with a new , already quite mature tree in a more suitable place - at their cost obviously…you never know you could get a more lovely year round interest tree instead of a rather dull one , and get a border makeover at heir expense whilst I was at it 🤣😉. It’ll cost them..but so will subsidence and an insurance policy that states they have trees within 3 metres.
but I agree, they were stupid to not negotiate this with you from the off. 🤷🏼♀️🙄