Row houses can have very few outside decorations (the roof counts as outside), I think 4 or 5 items. Each plantable spot in a planting bed counts as one decoration, so a small planting bed, with four plantable spots, counts as four decorations.
Yeah, I just really wanted to plant on top of my 2 story obsidian row They look so much cooler than the greenhouse row. I specifically got a 2 story obsidian row since it had a larger roof area