My Thujas at work are probably 40 feet tall and 20 feet across at the bottom. That should give you some idea.

At least they smell nice tho !!!

its nothing to do with your neighber thow you dont want to fall out. youve just got to worry wjhat it does to your house realy . if it grows big as said id keep it as far as you can from your footings . the trouble is in winter big plants/trees are fine but when it gets hot in mid summer the large shrub/plant can take all the moisture out of the soil making it shrink and potentioly making your house drop and get cracks . then it will need underpinning which is real expensive and youl have to cut the plant down . im assuming the plant in question is a chunky shrub as im rubbish at latin names . it would be fine if this is a climber as they dont have much of a root systems as such . saying that even a climber needs to be out from the house a couple of feet to miss the footings and lent in togrow up trellis etc . also for looks a large shrub or tree should have enough room to stop it squashing up to the wall anyway so you dont lose its natural shape .

@ Noseypotter. Thuja plicata is a large conifer, common name western red cedar. They make wonderful specimen plants (especially the 'zebrina' variety) in parks and large gardens but are way to big for the average garden. They also, like all Thujas, have very aromatic foliage which i absolutely love the smell of.

However, they are pretty good at being clipped back so maybe that's the answer before it gets too big.

I have 2 Thuja in my garden and they have barely grown in 4 years. I know that they do eventually grow to large trees but they are also excellent hedge plants. Annual pruning into shape (they topiary into large shapes as well) will keep them from growing into a 70 metre monster.

I would say that a hedge can be 3-6 metres away from a house and if allowing to turn into a tree (don't) they 40+ metres - both depend on soil and foundation type.

well clipped then you could go closeish to the house but of not its possible you should not have it at all then realy .

An absolute minimum of 45 feet away from any house or building, if you're going to let it reach maturity untrimmed.

Thank you to all that replied, I am going to cut back the Thuja's to a manageable height, say 6 foot. They are about 4 foot from my neighbours house wall.
This info was requested because their insurance people say my trees are taking all the moisture out of their clay foundations.

If they had asked me personally I probably would have taken then down completely, however no word has been spoken just a letter,( and I thought we were friends), so they are staying put for as long as possible.

Thank you all again.

i understand to a degree but arnt you just giving yourself more work ie cutting your own nose of etc . id rise above it and move on and cut them down . the worst thing you can have is bad neighbers .

Hmm, I'd take them out post haste, whether or not the neighbour has approached you directly - if they get subsidence, and a check on encroaching roots proves its from your trees, you'll be the one paying for the repairs next door, their insurance company will claim against yours.

like i say just not worth it . rise above it you wont change people but you have had a little hint of what there prepared to do .