I've done an accurate full-plans application for a chapel conversion
with sketch-up. Once you use a process of creating accurate component
parts, sketch-up seems quite happy with them accurately placed.

I've not much previous experience with architectural cad, but a lot of
years professional experience with electronics/chip design cad. So I
guess I'm well disciplined about not allowing errors to creep into the
model.

In the case of sketch-up that means, getting components accurately
dimensioned (even if only a box outline, that will be edited to add
more detail later), being very strict about making sure I'm working on
the right layer and great care with construction lines so everything is
placed where I think it is.

Generally I like the positioning inference (the
snap-to-significant-point), but I would like a single key overide.
Visual cues about which layer I'm working on would be good too.

It also seems to be possible to leave behind un-erasable artefacts if
one component touches another - and then you try to transfer one
component to a new layer. Seems to be a bug - and I haven't found any
solution - other than "don't do that".