Feasibility Study for an Accessory Dwelling Unit
This is a feasibility study we prepared for clients wishing to build one or more detached additional dwelling units (DADUs) in the rear yards of two houses they own in the south Seattle neighborhood of Georgetown.
In the first part of our feasibility study, we have researched the subject properties and the applicable zoning regulations to determine the potential for developing a DADU on the rear portion of the subject lots, labeled PARCEL A and PARCEL B. Below is a drawing of the subject property boundaries, including the adjacent PARCEL C and PARCEL D, which appear to be under the same ownership.
We have determined that development would be limited to one DADU on the rear portion of PARCEL A. Below is our analysis of the DADU development standards specific to this parcel.
SF5000 – One single family home per 5000 square feet (sf) allowed. Attached or detached accessory dwelling units (DADUs) are not considered a separate dwelling and are allowed, subject to the development standards from SMC 23.44.041 Table B summarized below.
PARCEL A PARCEL B DADU minimum (per SMC 23.44.041)
Lot Width 40’ 40’ 25’
Lot Depth 100’ 62’ 70’
Lot Area 4,000 sf 2,480 sf 4,000 sf
Minimum Lot Dimensions
PARCEL B does not meet minimum area (4,000 sf) or depth (70’) required for a detached accessory dwelling unit (DADU). An attached accessory dwelling unit might be allowed, but parking via access easement over PARCEL D needs further study to determine actual feasibility. One DADU on PARCEL A could be constructed with the following considerations:
Maximum Lot Coverage
For lots smaller than 5,000 sf, the maximum lot coverage is 1,000 sf + 15% of the lot area, which may include one-half the width of the alley for the purpose of this calculation (SMC 23.44.014.B & 23.44.014.D.1). Maximum total lot coverage of existing and new structures = 0.15 x (4000 + 400) = 1660 sf
Per King County Residential Data, the existing house on PARCEL A appears to be 1,060 sf, leaving approximately 600 sf available footprint for the DADU. Any decks > 36” above grade or eaves > 36” would also count toward the total lot coverage.
Maximum Rear Yard Coverage
DADU + any other accessory structures are limited to 40% of the rear yard.
20’ Front - DADU may not be located in the required 20’ front yard
5’ Sides - DADU could be located in required 5’ side yard only with easement creating permanent 10’ separation to any future adjacent structures
20’ Rear (lot is >125’ deep) measured from the centerline of the alley - DADU may be located in required rear yard; since rear P.L. abuts an alley no setback is required
Location of DADU Entry
May not face nearest side property lines, but OK to face alley.
DADU maximum gross floor area
800 sf, including garage & storage areas but excluding covered porches/decks < 25 sf
DADU height limit
15 feet base structure height + additional 4-7 feet, depending on roof shape
Minimum separation from principle structure
One off-street parking space is required for the accessory dwelling unit; it can be tandem with the parking space for the principal dwelling unit.
An owner with at least a 50 percent interest in the property must occupy either the principal dwelling unit or the accessory dwelling unit for six or more months of each calendar year as the owner's permanent residence. (SMC 23.44.041.C)