When does a tree become dangerous

[sfella]

Doesn't affect me in the slightest but looking out over a neighbours garden there is a conifer on the opposite side of them that is totally out of control. Just wondered what the score is on this wet Thursday!!

To put into context as I dont feel it's fair to post a picture of a neighbour, the tree is without exaggeration twice the height of the house (normal two story house with pitched roof), ground levels put the house slightly higher if anything than the base of tree. It is also at least 20-25ft wide.
I wouldnt want to sleep in there as the prevailing wind direction means if it comes down in high winds, chances are they are getting landed on. The land is owned by a local builder and has been for the past 25 years+ from looking at land registry, he keeps being refused planning to erect a house in a different place on the land to the existing barn so it just sits there. The bulk of the land is rented out and used as grazing but this bottom corner has been left and two conifers are the main features, one is much smaller for the moment.
Guess what I'm asking is what's the deal? Builder probably doesn't care, can they make him fell it? Etc