A rain-garden proposal should explain how its dimensions were chosen. Before comparing plant lists or excavation prices, ask for the drainage-area sketch and sizing worksheet. Wisconsin’s November 2018 homeowner manual, still linked from the DNR rain-garden page, provides a useful way to question those assumptions without pretending that a garden drawing is an engineered drainage approval.
Draw the Water Route Before the Garden Outline
The manual separates roof runoff from additional lawn runoff and explains how the contributing area changes with location (PDF p. 9). Have the designer mark each connected downspout and the part of the roof feeding it. A footprint measurement and an unexplained percentage are different levels of evidence.
Walk the proposed route together. Record whether water crosses a lawn, follows an existing channel or enters a pipe. Include a planned patio or gutter change in that discussion. Otherwise, the excavation contractor may build a garden for one drainage arrangement while another contractor sends it a different flow.
Ask Which Soil Result Selected the Table Row
Page 8 describes soil testing; Table 1 on page 10 varies the sizing factor with soil and ponding depth. Request the actual observation and chosen row, rather than accepting a soil label copied from a neighboring property.
For an illustrative 480-square-foot contributing area, a selected factor of 0.30 produces 144 square feet of ponding area. That arithmetic is an example, not a design for your address. Ask the designer to show what changes if the measured soil result or feasible depth differs. This makes the proposal easier to compare before anyone orders plants.
Check What a Longer Runoff Route Assumes
The smaller factors for locations beyond 30 feet assume water infiltrates on its way across the ground; the contributing lawn is also considered (PDF pp. 9–10). Distance alone therefore does not explain a smaller proposed garden.
If a contractor proposes piping a downspout directly to the garden, ask how that route affects the calculation. Request a sketch of the inlet and the intended destination of excess water. Put unresolved drainage questions back to the designer before authorizing a trench that could be difficult to relocate.
Specify the Measured Basin in the Contract
The manual distinguishes a level ponding area from the surrounding landscape shape (PDF p. 11). Ask the quote to identify the basin dimensions separately from decorative planting, edging and access work.
Keep the sketch, calculation and plant proposal under one revision date. Agree who checks the finished dimensions, repairs disturbed lawn and returns if the inlet erodes. Ask your municipality about any approvals relevant to grading or discharge at your property. The DNR educational manual does not establish that every proposed installation is permit-free.
Questions Homeowners Ask
Should the rain garden go in the yard’s wettest patch?
The manual cautions against an area where water already ponds (PDF p. 4). Have the cause assessed before treating that spot as the preferred infiltration location.
Does a larger plant bed mean a larger working rain garden?
Ask which measured area actually holds incoming water. A border or adjacent planting strip should not silently substitute for the calculated basin in the proposal.
What should I keep after construction?
Save the final drainage sketch, soil observations, selected sizing assumptions and maintenance responsibilities. They help explain the design if gutters, paving or planting change later.
Sources And Local Resources
Official sources checked October 17, 2026. Confirm current requirements with the authority responsible for your property.
- Wisconsin DNR and Standards Oversight Council: Rain Gardens, November 2018PDF p. 1 gives November 2018; PDF pp. 4, 8–11 match printed pp. 4, 8–11. PDF numbers count from the cover as page 1. Supports ponding-location caution, soil assessment, contributing-area method, Table 1 factors and level ponding area. Numerical 480 × 0.30 example is original arithmetic, not a source case.
- Wisconsin DNR: Rain GardensCurrent DNR landing page links the homeowner/landscaper manual and describes its runoff-management purpose; it is not a parcel permit determination.
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