Cost & Engineering - Over Detached Garage Room?

pbx2_gwApril 27, 2012

We're talking plans & costs with our builder/GC on our 3br-3.5 bthrs 1.5 story cottage style home with detached garage.

We were thinking of adding a 4th room + 3/4 bath over the detached garage =

Estimated to be about 300-400 sq.ft.

With a bathroom.

Staircase to go inside & up.

Builder is saying it could be a big cost around $25k+ because the HVAC & plumbing would be in a different 'envelope' vs. the main house - thus necessitating a sewer lines run down, over, underneath concrete to connect to main house - could be expensive.

Does this sound correct?

Not sure how 'valuable' this addition would be - since it is detached from main house - vs. having just an extra room in the main house would be...

We are squeezed for additional space in the main house plans due to structural reasons (High angled steeple roof lines 2nd floor) - so finding room could necessitate a costly change to the structural profile of the house.

However, we could accept maybe 1/3 of the garage cost & give up the bathroom.

So I guess the main question I am pondering is:

Cost of extra room+ bath over detached garage


Cost of bumping out some space in the main house.

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Costs vary by region of course, but our detached 2+ car garage with interior stairs and upstairs unfinished space was about $35k, about 4 years ago. (That doesn't include the cost of the driveway or any plumbing or HVAC, but we did put a separate electric panel out there in case we want to add more at some point.) So $25k sounds reasonable.

However... be sure to check your local zoning. Ours doesn't allow for plumbing in a detached garage (they want to avoid things like rental units); even a spigot for a hose outside is not allowed from the garage.

There are also considerations like egress (think you'll need more than one exit for the space to be considered safe) and also building an envelope between garage space and living space so that fumes don't carry.

Also, not sure if your garage is currently existing or to be built. Interior stairs will take up some space inside, so space for your car will be shortened by several feet, even if the staircase turns. (We handled this by bumping forward one half of the garage to be a few feet deeper than the other half. I wanted this anyway so that it wasn't one big flat expanse across the front, and it looks nice.)

To be honest, I think you'd be happier with an addition that's part of the house, especially if you'll use it frequently.

    Bookmark   April 27, 2012 at 2:03PM
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I doubt that 25k number is too far off. Unless you want to use it as more of a guest suite, I would add it to your main house. Eliminating the bathroom would cut the bulk of your costs out however. Depending on the size of the garage, you could do it with bonus room trusses. You would not need an elaborate heating/cooling system. A simple single head mini split hp would do the trick. They only run around $2k online.

    Bookmark   April 27, 2012 at 2:50PM
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Sophie Wheeler

Detached garages are already much more expensive than attached. Adding in living quarters (IF it's allowed0 will about double the cost. I think the quote is actually too cheap for all of the engineering that this will need. If you want to lessen the costs, attach the garage to the house and save at least 50K over a detached residence garage.

    Bookmark   April 27, 2012 at 3:08PM
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Yeah - we figured that those #'s were very rough estimates.

The living quarters ARE allowed in this development as some other houses have over detached quarters.

However, they ALL have no bathroom & essentially serves as a bonus room.

    Bookmark   April 27, 2012 at 4:30PM
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