GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Converting a Garage into a Guesthouse

Jayc123 | Posted in Energy Efficiency and Durability on

I am converting a detached garage into a guest house.  The garage is fairly new at about 10 years old.  The exterior finishes are in great shape.  It has a shed roof that has 2’ deep trusses and faces south.  We are adding solar and a going to use as little (recycled only) foam as possible.  I would like it to not look like it used to be a garage and to be net-zero when we’re done.  We will most likely use a single mini split to condition the space and a pair of Lunos e2 for venting.   I am looking for advice on how to proceed.  Thanks!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Joshua Salinger | | #1

    Hi Jay,

    Where are you located and what climate zone are you in? Also, what specifically are you looking for advice on?



    1. Jayc123 | | #3

      Hi Josh, I’m in N. Vermont Zone 6. My concern not wanting to cause moisture issues and I guess I’m concerned about adding too much insulation to the inside. I’m going to take a closer look to see what’s existing and how well the flashing details are holding up. I’ve had some suggestions to use aerobarrier to seal the space and treat the wall (with existing 2x4 studs) like a double stud wall adding another 2x4 wall inside. I don't want to cause any moisture issues in the wall. Thanks

      1. Expert Member
        Joshua Salinger | | #5


        Thanks for the information. You are on the right track in that you are paying attention to bulk water (flashings) and the air barrier. These are the two most important items to address. To a lesser extent, vapor and then thermal layers- in that order. You may want to consider a smart membrane on the interior walls for the air barrier as it also will act as the vapor control layer, which in a cold climate with a double stud wall approach will add resiliency. If you have a simple shed roof, you may want to create a site made baffle for ventilation at the roof and connect it to the low and high end soffit vents. Cellulose insulation has a buffering capacity which could help manage vapor and this could be used in the roof assembly. Be sure to not put any can lights or penetrations in the roof. You could also use a closed cell spray foam under the roof sheathing and create a hot roof assembly, depending on your stance on ccsf.

        If you go with the Lunos, know that anything beyond 600 sqft will require two pairs of these devices. Also, don't locate a unit in a a room where someone is sleeping as the back and forth, albeit quiet, can be annoying.

        Hope this is helpful...


        1. Jayc123 | | #6

          Thank you, Josh. This is very helpful.

  2. walta100 | | #2

    Seems like the first question is will your local zoning/government allow ADUs in your neighborhood?

    Are you prepared to rip out the concrete slab to install your drain pipes?

    Are you prepared to upgrade your electric service in the house for the new load of the ADU or will the ADU have its own meter?


  3. plumb_bob | | #4

    Will there be a municipal inspector involved? Some thoughts to consider:

    Planning related permits

    Certification of slab, radon requirements, plumbing (as mentioned above), tying into services, electrical certification, mechanical ventilation, egress requirements, parking, alarms (CO and smoke), spatial separation.

  4. andy_ | | #7

    I did something like this many years ago when I converted a detached two car garage into an office and storage/mini shop space. Not exactly an ADU, so permitting was pretty easy and the planning dept even gave me tips on how to keep it from needing any permits other than electrical.
    As far as making it not look like a converted garage, the obvious is to not have a garage door and a driveway going right up to it. You can landscape or put a patio/low deck over the driveway.
    The garage door space is a dead giveaway, but that too can be fairly easy to address. In my case I used two pair of outswing french doors but inset them about a foot and a half. That gave a nice cover from the weather and made the space look more intentional. Either side of the doors became built in bookcases inside.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |