Only illegal if the tyre tread is on show the rim can be, ie use stretch.13s on a golf look stupid even down 120mm theres still loads of arch gap. I'm running 16s et32 on a 20mm spacer, but going up to 35mm on the rear. All depends on the et, to get it flush or slight pike uou need an et of aboit 0 after you've added the spacer, so either have a wheel with an et of about 20 or a bigger spacer