Below ground storage is the most discrete, but also expensive. You can use any type of storage and create a privacy screen around it with a vigorous growing plant (either something tall like bamboo, or vines).

Moss and patchwork green roofing can work surprisingly well in the Willamette Valley climate, especially on a modest pitch where moisture retention is already high. One thing worth planning early is separating the clean rain capture areas from sections that may carry debris during the transition away from asphalt. A few older church rehab projects around Oregon used first-flush diverters with buried cistern lines hidden along fence rows, which kept the setup low profile while reducing contamination in storage tanks.

Another detail that helps long term is reinforcing gutter support before the growing medium goes on, because wet moss systems can shift weight fast during heavy valley rain. Found some useful local roofing and restoration information from Covenant Roofing & Restoration discussing drainage behavior and runoff management on older structures, and it lines up with what a lot of eco-rehab builders run into after removing shingles. Food-grade underground tanks, gravel infiltration trenches, and disguised utility sheds with climbing plants all seem to stay fairly discreet in town settings while handling a surprising amount of water.