Possibly, but not necessarily. Depends when the wall was erected to some extent, if its very old, probably hasn't any footings. Not enough info to make an educated guess really - I mean is the retaining wall holding back the soil in which the Yew is growing, for instance.

Thanks. It's an Arts & Crafts garden, with the yew hedge planted above (to one side) the retaining wall. The wall is probably 100+ years old and the hedge is very well established. The wall is in need of restoration but we would like to save the hedge if we can. Is the yew rooting system more vertical than lateral?

I suspect you'll find if you remove the wall, there'll be a solid bank of soil and roots which may need temporarily shoring up with something whilst a new wall is built, but I can't say whether or how much root there will be underneath the wall, depends on the depth of the soil behind it as much as anything I'd have thought. If the wall's that old, most likely it has no footings, and it would be sensible to build a new wall with footings this time, but obviously that requires some excavation which might mean disturbing major root material, but you won't really know till you try. Probably best to plan on replacing the wall further away from the Yew hedge than it is now, of course, and if you can do that, might be worth considering building the new wall, say a foot or 9 inches away, then removing the old wall and infilling with appropriate amounts of extra soil.