HomeMy WebLinkAbout2021-09-22_MHCACKeyIssue3Memo-Revised-Updated 2021 Middle Housing Code Advisory Committee tiA F
Key Issue #3 Memo: Stormwater
September 22, 2021 ~'
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ISSUE
Runoff and stormwater impacts of middle housing.
As densities increase with middle housing we anticipate removal of more landscaping and trees to make
way for more driveways, patios, and other features, unless the City limits development of these features.
The Committee was provided with direction to explore potential limitations on the development of
impervious surfaces for middle housing and single-family homes.
KEY QUESTION & POLLING RESULTS
The results of initial polling conducted via Zoom at the 4th Meeting of the MHCAC on September 15, 2021,
are summarized below:
Poll Question 1: Should the City limit development of impervious surfaces or create landscaping
requirements for both single-family development and middle housing to mitigate the impacts of increased
density?
• Yes, impervious surfaces should be limited by the development code—4 votes (40%)
• Maybe, impervious surfaces should be limited, but I am concerned about the impact on
development feasibility—5 votes (50%)
• No, impervious surfaces should not be limited by the development code— 1 vote (10%)
SUMMARY OF COMMITTEE DISCUSSION
Committee member comments at Meeting#4 related to Key Issue #3: Stormwater are summarized
below.
Polling results for question 1 indicated general support for the City to limit the development of
impervious surfaces for both single-family development and middle housing, with approximately 50% of
Committee members expressing support for limiting impervious surfaces while expressing concerns
related to the impact of these requirements on development feasibility, and another 40% expressing
support for limiting impervious surfaces within the development code outright. One Committee member
did not express support for implementing additional impervious surface limitations within the
development code.
Middle Housing Code Advisory Committee Key Issue#3 Memo Page 1 of 4
Committee members posed questions about what would be considered a pervious or impervious surface
under the code, with one member noting that the use of"pervious" pavers in driveways could be
undesirable. Another member expressed concern that impervious surface limitations could impact
accessibility for differently-abled individuals, as they could disincentive the use of accessible pathways,
ramps, or other features that would improve accessibility. Staff responded that it would be appropriate to
conduct a feasibility analysis to determine the extent to which an impervious surface requirement could
impact the ability to develop accessible middle housing units, in order to better "calibrate" the proposed
requirement.
Another member expressed concern that percentage-based impervious surface limitations could
disproportionately impact development feasibility on smaller lots, but that the percentage-based
requirements may be more appropriate on larger lots. Staff responded that this would also need to be
studied for feasibility, and that it may be appropriate to "calibrate" required impervious surface
requirements based on lot size.
There was also some discussion of specific site conditions that may necessitate impervious surface
requirements, such as steeply-sloped sites or sites containing soils that do not drain well. While some
members felt that impervious surface limitations would be most appropriate for sites with these types of
conditions, others felt that such limitations are needed on a City-wide basis.
Members also discussed the utility of larger trees in absorbing stormwater on-site, with one member
suggesting that retaining significant trees should be a component of the City's strategy related to
impervious surfaces.
KEY CONSIDERATIONS
Stormwater: City code currently regulates stormwater management through the Stormwater
Management Code (LOC Article 38.25). The primary objectives of these regulations are to protect the
water quality of Lake Oswego and to prevent negative impacts of a stormwater runoff on downstream
properties.
The stormwater code requires that new developments and redevelopment projects over a certain size
manage all stormwater on the lot. These requirements are triggered for any project that creates more
than 1,000 square feet of new impervious surface area. The code requires applicants to submit a site
assessment, stormwater system design, and other analyses to demonstrate compliance with the code.
Middle Housing Code Advisory Committee Key Issue#3 Memo Page 2 of 4
One limitation of the City's stormwater code is that it only applies to the creation of new impervious
surface area. In a redevelopment scenario, the code would allow for a site that has high impervious
surface coverage to be redeveloped with the same level of impervious surface coverage.
Staff notes that the existing land use code (development code) generally does not limit paving or
development of other impervious surfaces, such as driveways, patios, sport courts and pools, except for in
certain overlay districts (see Impervious Surface Limitations in Lake Oswego, below). To address this issue,
and to generally incentivize developments with lower impervious surface areas, some cities establish
limits on overall impervious surface coverage. These limits are usually applied as a percentage of the site.
One benefit of this approach is that it could reduce the amount of stormwater runoff that must be
managed on-site and reduce off-site stormwater impacts. One potential drawback of this standard is that
it may restrict the opportunity for property owners to add additional parking areas, patios, sport courts,
or other impervious areas. Additionally, if the percentage of impervious area permitted is set too low, it
may constrain the feasibility of developing middle housing by too greatly limiting the size of new
structures, and therefore requiring very small individual dwelling units—which may not be attractive to
build.
Impervious Surface Limitations in Lake Oswego: The City has already adopted certain limitations on
impervious surface area in two overlay districts—the Glenmorrie R-15 Overlay District and the Uplands R-
10 Overlay District.
Per LOC 50.05.001.4, properties located within the Glenmorrie R-15 Overlay District are subject to a limit
of 50% impervious surface area on a given lot:
No more than 50%of a lot shall be covered with any of the following elements:structures, patios,
paving or impervious walks. However, pervious decks and natural-appearing constructed ponds
shall not be included within this limitation. Where a paved area contains mixed nonplant and plant
elements, only the nonplant portions of the area shall be included within this limitation...
Per LOC 50.05.012.6, lots located within the Uplands R-10 Overlay District are also subject to a limit of
50% impervious surface area, as well as a limit of 30% impervious surfaces in the area between the front
of the house and the front property line:
a. No more than 50%of the lot may be covered with impervious surfaces.
b. The area between the front lot line and the nearest edge of the building footprint shall not be
covered by more than 30%of impervious surface.
Middle Housing Code Advisory Committee Key Issue#3 Memo Page 3 of 4
To address concerns about how proposed impervious surface limits might affect development feasibility
for middle housing, the City will need to test any proposed changes along with proposed lot coverage and
setback standards on "typical sites" to determine impact on development feasibility of middle housing
types.
The pros and cons of limiting development of impervious surfaces are identified below.
Pros of limiting impervious surfaces Cons of limiting impervious surfaces
• Potential reduction in the amount of • May restrict the opportunity for property
stormwater runoff that must be managed owners to add additional parking areas,
on-site patios, sport courts, or other features.
• Reduced off-site stormwater impacts • If limits are set too high they may
such as erosion, flooding and overloaded constrain the feasibility of developing
infrastructure middle housing by too greatly limiting the
• Reduced pollution via runoff size of new structures and therefore
requiring very small individual dwelling
• Potentially incentivizes remodels over units, which may not be attractive to build.
demolition
POLLING QUESTION: Should the code limit impervious surface area on all residential lots?
Response Options:
1. Yes, impervious surfaces should be limited by the development code.
2. Maybe, impervious surfaces should be limited, but I am concerned about the impact on
development feasibility.
3. No, impervious surfaces should not be limited by the development code.
Middle Housing Code Advisory Committee Key Issue#3 Memo Page 4 of 4