Hi Anne,
My sewing unit is a built into the wall affair, custom made by my deceased cousin in law, so rather precious to me now. He made it the same as a regulation kitchen counter top which I believe is about 24" deep. When I got my 12000 and put it on the counter it wasn't deep enough to open the arm, so my hubby took off the counter top and put about an 8" extension to the back, did a patch work on the arborite with a left over scrap and all is well. I'd even be OK with a couple more inches but don't really need that either - at least for the present.
I think the measurement of the machine and the open are is 27" so that is the area you need.
Cheryl - Saskatoon