Grand Rapids has made it more straightforward to pursue an accessory dwelling unit (ADU), but “more straightforward” still does not mean “simple.” The zoning path is clearer than it used to be, yet many ADU projects still hinge on practical realities: utilities, building-code requirements, and how the finished unit will be documented and valued at resale.
This article is Grand Rapids focused, using the City’s published ADU materials and ordinance text. If you are outside Grand Rapids city limits (Ada, Cascade, Byron Center, Kentwood, etc.), the concept may be similar, but the rules are different.
Note for Kentwood homeowners: Thomas Switek also serves as a Planning Commissioner for Kentwood, and Kentwood is actively working through what an ADU framework should look like locally. Until Kentwood’s approach is finalized, treat any Grand Rapids-specific guidance as non-transferable and confirm directly with the municipality where the property sits.
First: what counts as an ADU (and what the City is trying to prevent)
Grand Rapids describes an ADU as a secondary housing unit on a single- or two-family residential lot. It can be attached to the home or detached as its own structure. You will also hear “granny flat,” “in-law suite,” or “tiny house.”
A common point of confusion is the difference between an attached ADU and an attached two-family home (duplex). The City’s ADU FAQ notes that Michigan building codes do not differentiate between the two in the way many homeowners assume. In other words, the line between “ADU” and “two-family” is not just about having a door in a hallway. It becomes a zoning and use question, plus a size and configuration question.
Grand Rapids zoning basics: what is actually required
At a high level, Grand Rapids’ published ADU standards include the following:
- Quantity and eligible lots: Not more than one ADU per lot, and it must be associated with a single-family or two-family dwelling.
- Lot size: The lot must meet the minimum lot size of the applicable zone district.
- Setbacks: Attached ADUs follow primary-structure setbacks. Detached ADUs follow accessory-structure setbacks.
- Height: The maximum height for a detached ADU is 25 feet.
- Bedrooms: A maximum of two bedrooms.
- Rental limits: No rentals under 30 days, and the unit may not be used as a short-term rental (Airbnb and Vrbo style).
- Deed restriction: A deed restriction enforceable by the City must be recorded stating the ADU will not be conveyed separately from the primary dwelling unit(s).
Size limits: the FAQ and the ordinance PDF do not match
This matters enough to call out clearly.
- The City’s ADU FAQ (August 2026) states the zoning ordinance was updated in June 2026 to allow ADUs of up to 1,000 square feet, and that an ADU may never be larger than the primary residential structure.
- The City’s published Sec. 5.9.03 (Accessory Dwelling Units) PDF states a maximum ADU size of 850 square feet, with a formula cap of 40 percent of the primary structure gross floor area (or 400 square feet, whichever is larger), and an absolute ceiling at 850.
Practical takeaway: before you finalize plans or commit to a specific floor plan, confirm the current version of Sec. 5.9.03 with the City (or have your architect or contractor confirm it during early feasibility). That confirmation step is cheap insurance compared to redesigning after plan review.
Utilities are where “backyard ADU” budgets get real
The City’s ADU FAQ is direct about utilities not being an afterthought:
- Attached or internal ADU: a plumber is responsible for confirming the existing water service, meter, and sewer lateral are adequately sized. If a larger meter is required, additional connection permit fees can apply.
- Detached ADU: the City indicates a dedicated sewer connection is required, and a dedicated water connection is preferred. Water and sewer connection and inspection permits are required.
The City also publishes a Grand Rapids Water System standard detail (WS-10) for ADU sewer and water service connections. Among other notes, it emphasizes that the property is not allowed to be split (consistent with the deed restriction), and it lays out minimum expectations that can affect feasibility, such as sewer depth concepts, cleanout spacing, and minimum water service sizing.
Strategy note: if an ADU is being evaluated for longer-term rental income, utilities can be a major swing factor in whether the project pencils out. Price and scope the utility path early, then design around what is actually feasible, not what is convenient on paper.
Permits in Grand Rapids: what you actually apply for
For a City of Grand Rapids ADU project, expect multiple permits, not a single “ADU permit.” The City’s Development Center publishes a residential building permit page that includes an ADU building application and separate trade permits (electrical, plumbing, mechanical). Water and sewer connection permits may also apply depending on the project. The City also notes that to apply, you must be either the property owner or a licensed contractor.
At the state level, Michigan LARA’s building permit guidance explains the general framework: Michigan’s building code controls construction and related work, and permits are required before construction begins. It also notes that residential builders must be licensed, with certain exceptions.
Which code book matters?
Michigan LARA’s code book page lists the 2015 Michigan Residential Code as the current residential code and the 2021 Michigan Building Code as the current building code (effective April 9, 2025). In ADU planning, zoning is only one gate. Cost and timeline are frequently dictated by what the building official requires to issue a certificate of occupancy.
Building-code impacts the City is explicitly warning homeowners about
Grand Rapids’ ADU FAQ highlights several items that can materially affect cost and design. It states that attached and detached ADUs are treated as an additional dwelling and are subject to the Michigan Residential Code, and it flags requirements that may include:
- Separate air handling systems for the main house and the ADU.
- One-hour rated fire separation between the main house and ADU (walls and ceiling assemblies).
- Separate utility meters and electrical panels for the main house and ADU.
- Trade work (mechanical, electrical, plumbing) requiring permits and licensed contractors.
The FAQ also cautions that converting existing space, including a detached accessory structure like a garage, may require wall and floor alterations to meet fire separation and rating requirements.
Translation: the “convert the garage loft cheaply” idea can collide with compliance costs. The hidden costs are frequently in systems and assemblies, not finishes.
Renting an ADU in Grand Rapids: allowed, but not as a short-term rental
From a real estate planning standpoint, the City’s intent is clear in the published ordinance text and FAQ: ADUs can add housing and can be rented, but they are not intended to function as a short-term rental loophole. Grand Rapids’ published standards emphasize a 30-day minimum rental period and no short-term rentals.
The ordinance text also notes that if the ADU is not occupied as a residence by the owner, it must be registered and certified under Chapter 140 of the City Code.
If you’re buying or selling a Grand Rapids property with an ADU (or ADU potential)
From a transaction perspective, the cleaner the documentation, the smoother the sale. Two scenarios come up in the real world:
- Existing ADU that is permitted and documented: easier to underwrite and explain. Buyers, lenders, insurers, and appraisers may ask for basics like permits, legal use, and utilities setup.
- “It has a second kitchen” or “it’s basically an ADU”: this is where deals can wobble. A buyer may love the flexibility, but lenders and insurers may require clarity on what it is and whether it is legal.
A due diligence checklist that protects resale
- Zoning confirmation: verify the lot is eligible and the ADU configuration matches current standards (attached vs detached, setbacks, height).
- Size limit confirmation: confirm the current maximum size standard with the City due to the published mismatch between the FAQ and the ordinance PDF.
- Deed restriction: confirm it is recorded, or confirm the process and timing if you are building new.
- Rental compliance: if it has been rented, confirm registration and certification requirements were handled correctly, and confirm you can operate within the 30-day minimum rule.
- Utilities path: especially for detached ADUs, understand sewer and water service connection expectations early.
- Permit trail: collect building and trade permits and final approvals. This supports safety, marketability, and smoother underwriting.
Resale and appraisal: where ADUs can help, and where they can create friction
It is risky to promise value premiums because the market does not automatically reward every dollar spent. A well executed, legal ADU can improve marketability by adding legitimate flexibility, including separate living space and longer-term rental potential that aligns with the City’s rules.
Two issues that can undercut value are:
- Ambiguity: if buyers, lenders, insurers, or appraisers cannot get comfortable with legality and documentation, the ADU can become a point of negotiation instead of an asset.
- Mismatch with the property: an ADU that is out of scale, under-served by utilities, or clearly nonconforming can create more friction than upside.
Recommendation: a short feasibility pass before full design
If you want to build an ADU in the City of Grand Rapids, consider a short feasibility pass before paying for full design:
- Confirm zoning eligibility and the current size limits with the Planning Department.
- Sketch the utilities path, especially sewer for detached units, and validate it with qualified trades.
- Have a contractor price the code-driven items early (HVAC separation, fire separation, meters and panels) so the budget reflects compliance realities.
If you want a second set of eyes from a resale and transaction strategy perspective, we can help you pressure-test an ADU plan against documentation expectations, financing and appraisal questions, and the practical constraints that show up during due diligence.
See more Switek Insights articles here.
Note: This article is informational and is not legal, engineering, or code compliance advice. Always confirm current requirements with the City of Grand Rapids (or the applicable municipality) and the appropriate licensed professionals for your project.