If you are planning to build raised beds I presume with either wood or stone, you would have to build up the down slope higher than the up slope to get a level bed. The slope doesn't matter. I used 4X4's, when they were cheaper to raise my beds because my house sits downhill because I detest planting on slopes.

Here is a link that might be useful: Propagating Perennials

We have that much slope or more, and we've terraced it into beds. The pathways get slippery in the winter, but the plants in the beds don't mind a bit.

Most of the beds are retained with poplar boards, and they hold just fine with simple stakes.

Here is a link that might be useful: my website