Replacing impervous paved surfaces with pervious alternatives can count towards being more nearly conforming. Calculate what your impervious area is now as well as what it will be post construction. If you can off-set the area of home expansion (new impervious) with an equal area of paved surface conversion (no longer impervious) then you do not need an engineer regardless of impervious percentage. The engineer requirement only kicks in when there will be both more than 30% impervious area and the project will result in an increase in the impervious area.
Another thing to consider, look to convert things like patios and walkways to pervious surfaces first. The reason is that areas that will not have a great deal of weigh and loading on them only need 8 -10 inches or so of base material under them. This is in part because they are at lower risk of compaction than areas that receive greater loading such as parking areas which should have about 36 inches of base material under them.
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