Cost & Engineering - Over Detached Garage Room?

12 years ago

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

vs.

Cost of bumping out some space in the main house.

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Comments (4)

chicagoans
12 years ago
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.
PRO
Epiarch Designs
12 years ago
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.
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Hollysprings, you made the following statement "You can't have bathrooms in two different structures share the home's main sewer drain." Can you show any zoning/building laws that agree with your statement? I believe you statement is not true. I just built a detached guest house and the bathroom was tied into the main home's existing sewer drain. All the guest houses and pool houses in this area are hooked up in similar ways to sewers or septic systems. If the structure is part of the main property and not a second home/apt that might be rented out then it is all hooked up together. They actually don't want separate hook ups with different meters because that would make it easy for some homeowners to rent that space out, which the town does not want at all.
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I don't see a problem as long as framing will allow it. but from a heating & cooling perspective these areas can be difficult to heat/cool. there are many threads about family rooms over garage (frog) with people complaining about the difficulty to condition these areas. problems are...heat gain from being so close to roof, heat gain from walls of room shared with attic space, air movement through walls, over and under the room. air sealing & insulating is key to making these spaces comfortable. these rooms are surrounded by extreme attic temps. so additoinal care is needed to deal with this gain. either taking the time to insulate then use foam sheathing boards to air seal...or foam. while spray foam costs are high...labor to air seal with foam sheathing boards quickly adds to material costs of this detail orientated install. foam sheathing, tape for seams, button cap nails to install & caulk to air seal. I'd look into costs of foam..spray foam the roof line & the west facing wall. since the garage is detached... minisplit is most costly. but a heat pump minsplit would be very nice. less costly would be heat pump p-tac.. noisy & hole into wall would need to be cut. least expensive would be window unit with heat pump... not a/c with electric strip...very expensve to operate in winter...not much cheaper in summer. will plumbing be added for bath or mini kitchen area? plumbing is expensive, but if you are going to do it.. make a full bath. and of course...pick windows with low e coatings & inert gas if on west side.
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We have a two story foyer and a regular 10' ceiling in our great room which is in view of foyer...it looks fine. As far as whether to do the playroom over the great room vs over the garage, for me there would be several factors to take into consideration. 1. Like cpartist said, a lot would depend on the floor plan upstairs. 2. I like the 16 x 18 dimensions better than the 12 x 25....I don't think long narrow rooms are as easy to use or decorate. Plus, I would imagine the natural light would be better in the 16 x 18 space. 3. To me, the only drawback to having the playroom over the great room would be the thumping from activity. My ds plays vigorous indoor bball games with friends, and my dd is a tumbler. I am grateful the floor space is well insulated, but still hear the occasional thump. 4. Could you make the space over the great room, and leave the space over the garage unfinished? That way you leave all of your options open. I really think you will prefer not having a 2 story great room, regardless.
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User
12 years ago
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.
pbx2_gw
Original Author
12 years ago
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.

Cost & Engineering - Over Detached Garage Room?

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chicagoans

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Cost & Engineering - Over Detached Garage Room?

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chicagoans

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pbx2_gw
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