Welcome to the Accessory Dwelling Unit ultimate FAQ guide! I've compiled all the answers from my previous two posts, added more questions and updated things for a few recent code changes. You can find the original two posts right here:
Montgomery County ADU FAQ Part 1
Montgomery County ADU FAQ Part 2
You can use these links to skip ahead to the answer you need. Have a question not addressed here? Leave a comment below!
General Accessory Dwelling Unit requirements
What is an Accessory Dwelling Unit?
What permits do you need to build an Accessory Dwelling Unit?
What is an Accessory Structure?
Can you build an ADU but just call it an Accessory Structure?
Attached vs Detached ADUs
Can you attach the ADU with a breezeway?
Technical Accessory Dwelling Unit requirements
How big can an ADU be?
Can an ADU be two stories?
What are the ADU parking requirements?
Can I turn my garage into and ADU?
Can I put an ADU above my garage?
Can I have a garage and an ADU?
The BIG questions
How much do they cost to build?
How much do architects charge for an ADU?
Can I buy a plan set online?
Can Chat GPT do my zoning analysis?
Permit Requirements
What drawings are required to build an ADU?
GENERAL REQUIREMENTS
What is an Accessory Dwelling Unit?
An Accessory Dwelling Unit (ADU) is a complete, independent living space on your property or within your house that is separate from your primary residence. Officially known as an Accessory Apartment and sometimes informally known as a Granny Flats or In-Law Suites, an ADU is basically an apartment on your property. You as the homeowner can live in the house OR in the ADU, but your primary residence must be on the property. You can only have one ADU on a property.
What permits do you need to build an adu?
A bunch. You will be dealing with multiple government agencies if you want an ADU on your property.
To build the thing you'll need a building permit from the Montgomery County Department of Building Services (DPS). This will include reviews of architectural, structural, energy efficiency, zoning and stormwater management. More about that in the required drawing section below. Once the ADU is under construction you will need electrical, mechanical, fire protection and maybe some other trade permits.
Licensing of, and the application for, the ADU itself will be handled by the Montgomery County Department of Housing and Community Affairs (DHCA).
Those are main two. You may need further permits, licenses, approvals etc. if you're in an historic district, a municipality such as Takoma Park, or an HOA or a few other special areas within the Montgomery County.
WHAT IS AN ACCESSORY STRUCTURE?
Any structure you build on your property that's not your primary residence is an Accessory Structure. This includes garages, studios, sheds, gazebos, etc. If you want your accessory structure to be an actual apartment it has to meet the standards of an Accessory Dwelling Unit. If your goal is to just build a home office in your back yard you will have fewer standards to meet. Send me an email for more details.
can you build an accessory structure but use it as an adu?
No. An ADU needs to be a fully-functioning apartment, which means it requires cooking appliances. You can not get a building permit from DPS that shows a second kitchen on your property (either within your house) or in an Accessory Structure without an accompanying letter from DHCA that an ADU application has been completed. You CAN create an accessory structure with everything BUT the kitchen if you do not intend to use it as an ADU. Zoning requirements are slightly different for ADUs and Accessory Structures, so it may be difficult to change your mind later if you decide to convert the Accessory Structure to an ADU.
Attached adu vs detached adu
There are two flavors of ADU; Attached and Detached, with some pros and cons to each.
A Detached ADU is an accessory structure out in your back yard. They are completely separate from the house, offer more design freedom and are high on the cool factor. They're also generally more expensive per square foot, are more restrictive in size and they require fire sprinklers and stormwater management, which are big expenses.
An attached ADU is an addition to your house. They are allowed to be bigger, you don't need to add sprinklers, you may be able to avoid drainage requirements (though unlikely if you want to max out your size) and it's generally easier to tie into existing utilities serving your house. You'll pay more in design fees because now you need to coordinate the addition with the entire house and it's sometimes difficult to max out the size and not reconfigure existing spaces you might not have intended to be a part of the project.
Consider a maximum size attached ADU is 1,200 sf…that's a 30′ x 40′ footprint, which is huge and might not fit on your site. You could also do 600 feet on each of two stories, but now we're getting into design, and that's part of the part I get hired to do!
Can you attach an adu with a breezeway
Yes, but not the way you're thinking. An attached ADU must be connected to the house via actual indoor conditioned space. Architecturally referred to as a hyphen, and informally (and somewhat incorrectly) as a breezeway, it is a great design tool which pushes the addition away from the main house, thereby reducing the connection point between the two, making that connection easier from a design and construction standpoint. That connection piece is a great place for a mudroom, laundry, powder room etc, but it is INSIDE space…not just a covered walkway. Check out the Seven Locks project on my home additions page for an example of a hyphen. The breezeway can be part of the principal residence or part of the Accessory Dwelling Unit, but you will need to provide fire separation between the two, which will end up being a design decision about how the space is used and accessed.
TECHNICAL REQUIREMENTS
HOW BIG CAN AN ADU BE?
An attached Accessory Dwelling Unit can be up to 1,200 sf. It can be larger if it is being created in a basement that already exists.
A detached Accessory Dwelling Unit is limited to the lesser of 1,200 sf, 10% of the lot size or 50% of the primary residence footprint. Every project is different, but usually the footprint thing is the limiting factor. Zoning setbacks are more restrictive for detached ADUs and for many properties it can be difficult to build more than 576 sf. If you send me a property survey I can tell you pretty quickly how big you might be able to go. Note that ADU size limit is Gross Square Footage, meaning the sum of all floors. The principal residence 50% limit is based on the footprint, meaning the outline of the house as viewed from above.
can an adu be two stories?
Technically yes, but it's probably not how you envision. The first limiting factor is the overall size. If your site allows a 700sf ADU, that is gross, so 700sf total, not on each floor. Stairs take up a lot of room, and you may not want to give that space up.
The second limiting factor is height. A detached ADU can be a maximum of 20 feet tall with an average height of 15 feet. You can't do two full height stories within that limit. The second story can be built within a dormer, but that floor space counts toward your limit and must be smaller than the area on the first floor.
If your first floor is under 400sf you may be eligible for the Tiny House appendix to the code, which will allow a much steeper stair an a loft space which does not count toward your gross floor area. The loft is limited to a 6′ – 8″ ceiling height, so again, not a full height space.
what are the adu parking requirements?
You will need to provide one parking space for the ADU in addition to any parking already required on your property (usually two spaces) and a lighted path from the parking space to the ADU. If you live within one mile of metro you don't need to provide parking. If your property does not have an existing driveway you may be able to get a waiver for the parking requirement.
CAN I TURN MY GARAGE INTO AND ADU?
Maybe. Your garage almost certainly does not meet the current ADU setback requirements. If you convert the structure exactly as it is, you may be grandfathered in by right. If you make any changes at all…roof height, window openings or various others, you will need a variance that will probably be approved, but that's not a guarantee. I'm also personally skeptical of the ability to adequately insulate, air-tight and waterproof an existing garage.
CAN I PUT AN ADU ABOVE MY GARAGE?
Yeah, but probably not likely. See height requirements above and the variance thing in the question before this one. I don't think this one is going to happen for you.
CAN I HAVE A GARAGE AND AN ADU?
Yes! Provided you comply with all the setbacks and lot coverage limitations, the size limits on Accessory Structures and Accessory Dwelling Units are calculated independent of one another, so you could theoretically max out the size of both.
The big questions
HOW MUCH DOES AN ADU COST?
Expect to spend at least $250,000 in construction costs for a detached ADU. They are very expensive per square foot. They require plumbing to make it to your back yard and to connect with the sewer. They require fire sprinklers, which often requires an upgrade in your water service line. They require stormwater management. A builder still needs enough profit to make it worthwhile.
HOW MUCH DO architects charge for adu plans?
Ben Norkin Architecture will start in the mid to high teens for a detached ADU and low twenties for an attached ADU, though my proposals always include three possible fees depending on how involved you need me to be throughout construction.
More on this soon, but charging for “a set of plans” though a common question, is not an accurate description of architectural professional services. My fees include architecture, and structural engineering and civil engineering and coordinating permits with at least two separate government agencies and helping you solicit and compare bids and various other services we can talk about if you're serious about building one of these things.
Also not the cheap option. I'm the exceptional option!
can i buy an adu plan set online?
You can try, but in my opinion you are not going to save a lot going this route, for a lot of reasons. I can already tell this answer is going to be long and ranty, so let me say that I have no moral objection to plan sets and I'm not offended by people wanting to use them. I have friends who sell them and have considered doing the same.
The problem with plan sets is that they try to commoditize a professional service. Plan set websites want you to believe that all you are getting from an architect is a set of drawings, and why pay 10X the price when you can get the same thing for $2,000. It's obviously not the same thing. It's like buying the Friday's branded buffalo wings at Giant. Sure, it might sorta be related, but it's not the same thing as going to happy hour.
So here are the possible issues you'll run into with trying to use a plan set to get a building permit for an ADU.
- You still need to hire a structural engineer to sign off on the drawings and create a wind bracing plan (likely not included in your plan set)
- You need someone who can do a zoning analysis, which might be a Structural Engineer who provides permit drawings, or a Civil Engineer
- You need someone who can and will edit the plans to meet local code requirements and respond to comments from DPS
- You need a drainage plan, so you need to hire a Civil Engineer
- If you want any customization of the plan set you're paying for that in addition to the cost of the plan set
- You need to know before you purchase that your plan set is going to meet zoning requirements. You can't get a variance just because you want a taller ADU
While there is no requirement to hire an architect, we are well positioned to handle all those issue. I think it's worth it to hire an actual professional to guide you through the process, because on top of all that boring stuff handled for you, you also get a completely bespoke design done exactly how you want.
If you already have a plan set, or are super motivated to try to do it all yourself you might be interested in my plan review service.
can chat gpt do my zoning analysis?
No.
PERMIT REQUIREMENTS
WHAT DRAWINGS ARE REQUIRED TO BUILD AN ADU?
The more the better, but if you want the bare minimum permit requirements, I can try to do that here.
Every project is different, so here's a super simplified list that will get you most of the way there.
Architectural Drawings for ADUs
Floor Plans
Architectural Floor Plans for an ADU need to show the size of the building and the rooms. Provide all dimensions. You need to show the location of doors and windows. Note tempered windows where required. Bedrooms should meet minimum code dimensions and have an egress window. Kitchens need to have cooking appliances, a sink, a fridge and cabinet and counter space. You will also need to show compliance with energy efficiency code somewhere. See my post about how to comply with the 2021 Energy Efficiency Code.
Sections and Elevations
You need to show a section of the building from foundation to roof ridge that shows the structure, wall and roof assemblies. Show all major structural elements. Show window and door location and materials.
Schedules
You'll need to include a list of the doors and windows used in the project.
STRUCTURAL Drawings for ADUs
Foundation Plan
Show the foundation walls or piers. Show the footings. Note any required reinforcement and local design parameters
Framing Plans
Note all joists, rafters, beams, headers, ridge boards, trusses, connections, etc.
Wind Bracing
Show braced wall lines, wind bracing method, dimensions of braced wall panels, if required. Provide details engineered sections or code references for IRC-compliant bracing methods.
ZONING Drawings for ADUs
Site Plan
Show the property boundaries, note all metes and bounds. Show the existing house location and size. Show all required setbacks and proposed setbacks. Show the proposed ADU, whether attached or detached. Provide calculations for lot coverage and ADU size. Show the on site parking if required.
Zoning Elevation
Calculate and show the average grade at the front of the house (attached ADU) or at the front of the ADU (detached ADU). Show roof height.
CIVIL ENGINEERING FOR ADUS
Drainage Plan
Your ADU likely requires a stormwater management drawing
Other Required Drawings
You may need a Sediment Control plan. You may need a right-of-way restoration permit. This is site, project and location dependent.
