Answer: It is a good idea to "transition" soil from one type to another. Plant roots will cross over into another soil type better if the interface is not so sudden. I would place about 4" of the new soil in the bed area, rototill or spade it into the native soil to mix them a bit, and then add another 4-6" and repeat the mixing of the surface 8" or so. It would certainly be a good idea to mix some well decomposed compost (or composted bark) into the new topsoil you are adding if it doesn't already have some included. A 50:50 mix is great if you can get that much. Your beds can be about 10-12" high for most annuals and perennials. If the lower clay layers are truly impervious, I would suggest 12-16" for the shrubs.
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