Zoning: Accessory Dwelling Units
Status
UPDATE 2024-11-05: Updated to include the applicability of table 5.2.08.G.1 related to maximum ADU size.
UPDATE 2024-04-23: On 2024-04-23 the City Commission adopted a collection of zoning reforms which included significant changes to Accessory Dwelling Units. All documents and commentary prior to this date should be disregarded.
Disclaimer & Purpose
This is not an official document; this document is not endorsed by the Grand Rapids Planning Commission, nor are its authors qualified to present legal opinions.
The purpose of this document is to provide an introduction to the regulations related to Accessory Dwelling Units in the city of Grand Rapids, MI as of late May, 2024. Within the Zoning the topic of Accessory Dwelling Units are directly addressed in section [5.9.03]. Other sections of the code may still impact the development of ADUs so we have attempted to cross-reference the most relevant sections.
Zones & Abbreviations
If you are interested in building an Accessory Dwelling Unit the first step is to determine your "Zone District". This Zone District will determine which criteria apply to your project.
The vast majority of lots in the city exists within exactly one Zone District. A Zone District is titled as a combination of a neighborhood type [TN, MCN, MON] and a zone [LDR, MDR, TOD, etc...] For example "TN-LDR" is a "Traditional Neighborhood Low Density Residential" zone, and "MCN-MDR" would be a "Mid-Century Mixed Density" zone. You can find the zoning map here.
| Abbreviation | Definition | Comment |
|---|---|---|
| TN | Traditional Neighborhood | |
| MCN | Mid-Century Neighborhood | |
| MON | Modern Neighborhood | |
| LDR | Low Density Residential | This is the vast majority of the city. |
| MDR | Mixed-Density Residential | |
| TBA | Traditional Business Area | |
| TOD | Transit Oriented Development | |
| CC | City Center | |
| TCC | Traditional City Center |
If you determine that your lot is split between zones [very rare] you should contact the Planning Department to determine what that means for you.
There are also "Special Districts" and "Overlay Districts". These do not appear to impact the development of Accessory Dwelling Units.
The ADU Q&A
Q#1.) Can I build an Accessory Dwelling Unit in my zone.
If you have a property in a and LDR or MDR zone, regardless of the TN, MCN, or MON prefix the answer is yes. Accessory Dwelling Units are a permitted use in all residential zones with a single occupancy or duplex (two unit) primary structure provided the lot meets the minimum size of the district.
If your property is located in a CC, TCC, TBA, TOD, C, or NOS zone your ADU will required Directory Review (DR). Director Review does not involve a public hearing.
Q#2.) How many Accessory Dwelling Units can exist on a property?
One. Both an ADU included within the primary structure and one within an accessory structure count towards the total limit of one. [5.9.03.A]
Q#3.) How large does my lot have to be to build an accessory dwelling unit?
An ADU is permitted on any lot which meets the minimum for size for its district [5.9.03.C]. Note that requirements for green space and minimum setbacks must still be met.
Q#4.) What is the height limit for an ADU.
An Accessory Dwelling Unit in an accessory [detached] structure may be 25 ft. tall [5.9.03.G].
An ADU within the primary structure cannot cause the primary structure to exceed the height of the zone district.The primary height for a zone distinct is described in the following table [5.5.07.A]:
| Zone | TN | MCN | MON |
|---|---|---|---|
| LDR | 35ft. | 35ft. | 35ft. |
| MDR | 45ft. | 45ft. | 52ft. |
The height of the structure is not measured to the peak or highest point. Height is measured to the midpoint between the peak and the eave. (5.2.06.A.2.)
Q#5.) Where may an Accessory Structure containing an Accessory Dwelling Unit be located on the lot.
The dwelling unit may not be located in the front yard [5.2.08.H]. Otherwise the location is principally limited by setback requirements - see Q#9.
Q#6.) What is the maximum occupancy of an Accessory Dwelling Unit
An Accessory Dwelling Unit is limited to two bedrooms. [5.9.03.H]
Q#7.) What are the limitations regarding the use of an Accessory Dwelling Unit?
- An ADU which is operated as a rental property may not be leased for a term of less than 30 days or to more than eleven different parties in a single calendar year. [5.9.03.I]
- Construction of an Accessory Dwelling Unit will involve the creation of a deed restriction on the property which will prohibit subsequent separation of the Accessory Dwelling Unit from the primary structure. [5.9.03.K] The cost for filing a deed restriction is $30; this is performed via the counties' Register Of Deeds. There may be other feeds related to drafting the deed restriction document.
Q#8.) Is it possible to re-purpose a mobile home or recreational vehicle as an Accessory Dwelling Unit.
No. The restrictions on re-purposed items as accessory structures - including as Accessory Dwelling Units - is very comprehensive [5.2.08.H.1]. It specifically prohibits mobile homes, trailers, railroad cars, and containers.
Tents are also prohibited as Accessory Structures; one can reasonably assume this would include Yurt type structures.
Q#9.) What dictates the minimum and maximum size of an Accessory Structure containing an Accessory Dwelling Unit.
The minimum is straight forward; the minimum size is 400sq/ft [5.9.03.F.2].
The effective maximum is constrained by a variety of factors. Principally these are green space requirements, set-backs, and proportionality.
Green Space
Accessory structures are subject to the minimum green space requirements. This limits how much of a parcel can be covered by structures or impervious surfaces
Minimum Greenspace [5.5.06.A]
| Zone | TN | MCN | MON |
|---|---|---|---|
| LDR | 40% | 50% | 60% |
| MDR | 30% | 40% | 50% |
There may be provisions where some covered or otherwise used areas may still be counted as green space - such as the use of porous asphalt in a parking space. Special cases should always be discussed with the city's planning department during the design phase of the project.
Set Backs
- At least 6ft from the primary structure. [5.2.08.H.1]
- At least 3ft from the side lot line. [5.2.08.H.1]
- At least 3ft from any other Accessory Structure. [5.2.08.H.4]
- An Accessory Structure must be set at least 3ft. from right-of-way of an alley. [5.2.08.H.4]
- City fire code requires special provisions if a wall is nearer than 5ft. to the property line [5.2.08.H.2.c].
Exemptions
There is a list of features whose footprint is not included in the footprint of the Accessory Structure [5.2.08.5] which includes the following items that may possibly be relevant to an Accessory Dwelling Unit: solar panels, decks, and patios.
Proportionality
The floor area of an Accessory Dwelling Unit may not exceed 40% of the gross floor area of the primary structure, or 400sq/ft, which ever is less. A maximum size of 850sq/ft. [5.9.03.F.2] applies regardless of the 40% ratio.
Your primary structure's gross floor area is likely larger than your "square footage" as the square footage frequently referenced is "Residential Square Footage" and does not include space below grade (the basement) - if your basement ceiling is at least 7ft high you may include that space in your calculation regarding the 40%. The 40% boundary is not a significant barrier to ADU development. Under the 40% rule any lot with a primary structure over 1,000 sq/ft gross floor area can meet the minimum 400sq/ftsize. The normal size range for ADUs in other cities is 500sq/ft - 650sq/ft which corresponds to a primary structure size of 1,250 sq/ft - 1,625sq/ft; this is the range of average home sizes in Grand Rapid's traditional neighborhoods.
The definition in the code [5.16.F] of Gross Floor Area is:
The sum of the gross area of each story of a building measured from the exterior limits of the faces of the structure, or from the centerline of walls separating two (2) buildings, including all areas of a building having a height of more than seven (7) feet, as measured from the floor to the lowest point of the ceiling. Measurement of GFA does not include areas used exclusively for vehicle parking or loading and, in industrial areas, storage sheds with less than one hundred and fifty (150) square feet of space, bunkers, electrical substations, smoking shelters, instrument shelters, and similar enclosures.
The table for provided in the Zoning [5.2.08.G.1] describing the maximum sizes for Accessory Structures based on lot size also applies; this is a change since the zoning reforms passed in 2024.
| Lot Area (sq ft) | Max Size (TN) | Max Size (MCN) | Max Size (MON) |
|---|---|---|---|
| Less than 5,000 | 624 | 624 | 832 |
| 5,000 - 7,499 | 832 | 832 | 936 |
| 7,500 - 10,999 | 936 | 936 | 1,000 |
| 11,000 - 21,999 | 1,000 | 1,000 | 1,200 |
| 22,000 | 1,200 | 1,200 | 1,500 |
So taking into account set-backs and green-space requirements the maximum is 40% of the gross floor area of the primary structure or the maximum of 5.2.08.G.1, whichever is less, with a minimum of 400sq/ft and a maximum of 850sq/ft.
Q#10.) Do I need to provide an off-street parking space?
No. [5.10.04.C] The creation of an ADU goes not impact the parking requirements of a property.
Remember that a Single Unit Primary Structure [aka: a house] is required by Zoning to itself have two off-street parking spaces. Many lots in the city, particularly in traditional neighborhoods, have zero existing off-street parking spaces. Now that you are adding an ADU to the lot - a significant change - this deficiency can be called into question [but likely will not]. Fortunately these requirements for the existing structure are potentially waivable. If you are replacing a "garage" which is currently not suitable for parking due to size or low door height, as is the case with many old carriage house garages, be certain to point that out so that it is not counted against you in how the initial amount of parking is considered..
Q#11.) How may an ADU in an Accessory Structure attach to city water & sewer.
Tthe ADU must be connected to municipal water and sewer; this is required by the Residential Building Code. Off-grid dwelling units are not permitted.
There is no requirement by the city that the ADU have a connection to water and sewer independent of the primary structure. However, the text of the Michigan Building Code of 2015 is vague, stating [this text is from a correspondanc with a Grand Rapids Utility Engineer]:
Under Michigan Plumbing Code, two structures may share a sewer lateral as long as they are under the same ownership and on the same parcel. If ownership or the lot is split, sewer separation is required at that time. The wye to the 2nd building must be outside both structures and the sewer lateral must run outside the 45 degree bearing plane of any structure. If you are using a pressurized system with a pump, Larry Olson in Utility Engineering will need to review and approve the pump and proposed location of the private forcemain before work may begin.
A dedicated water service is required for each structure from the main to the meter. On public property, we require a minimum 1" water service made of type "K" copper. We recommend copper for the entire service, but any material accepted under Michigan Plumbing Code may be installed on private property.
We are monitoring two existing ADU projects to see how the water and sewer connections end up being accomplished.
Q#12.) Are they are any special requirements for a for-rent ADU?
No. The same rental certification process applies to an ADU as any residential property; the required information, including an inspection check-list, can be found here.
The most notable requirement for rental properties beyond a state of good repair is the requirement for 10-year sealed lithium battery or hard-wired with battery back-up smoke detectors These must be present in every bedroom, outside sleeping area(s), and on every level. [9.832, 9.833] The best way to address this is to have hard-wired smoke detectors installed in the unit during construction, even if you do not initially intend for the unit to be for-rent.