To ensure this won't look too weird, get a competent local architect involved.

Thanks, Mark. We are speaking to a few to get ideas. Are there any aspects to note that they could help with?

When you said your husband works on cars, I recall one vacation house which was terrible in that a neighbor's running machinery ruined some peace. I can't imagine a renter liking that.

I've seen good results with an added master suite, with typical larger bathroom and closet. I wonder about the office, etc... on second floor? Have acoustic expert advice!

Studio's balcony could be on other wall, other side of your master suite's ground patio for privacy. Not a 'fussy' deck with posts, but a balcony which I'd think would increase appeal.

@everdebz - yes, two sides will have a great view and the car mechanic work is more classic car tinkering than industrial car garage.

Offices/rooms on the opposite of the car size doors will have a great view and we'd put the stairs on the same side as in the photo but going up from the non car door side... does that make sense?

Adding a whole second house to an existing house and complying with HOA regs is going to be cost prohibitive. Unless you’re prepared to buy your house all over again in the addition.

As much as you might want to because it's fashionable, putting a Millennial-style Neo Edwardian carriage house turned garage on the same lot as your house is going to make your modest-cute 1970s modular home look like a double-wide you are living in until you tear it down and build the big house.

You can build a garage that's bigger than the house. American house have had big barns behind them since forever.

It just can't be fancier than the house itself. Do a nice plain shingled rectangle with a discreet entrance to the rental unit and don't hang a bunch of gingerbread off it, because you can't add any of that stuff to your actual house down the line and have it look right either.

Work with a vocabulary that complements your perfectly fine modest modular, not with one that insults it.

@palimpsest That was a very thoughtful commentary and we apprecit. Any pictures or links that could help us get on the right foot?

Sometimes with online house plan sites they have plans that go back to mid century, but you could literally start out with a more current plan you like and take off the ornaments and modify the roof pitch to make it more compatible with your house and use window styles and lighting compatible with your house. And if you are keeping the deck on your house, match the stairs to the unit to the deck stylistically (adjusted to current code of course. Yours will be a matter of subtraction rather than addition of details.

https://www.houseplans.net/floorplans/312500029/traditional-plan-420-square-feet-1-bedroom-1-bathroom

I don't think the neighbors will appreciate any 2-story set-up that may block their views to the lake. This one looks to be more compatible with your home's style too. Could it be attached via the breezeway?

A rental unit, whose only access is via exterior stairs, is not going to command a good price (snow / rain).

I like the one decoenthusiaste posted. It's only a studio but maybe it could be expanded.

If you have to connect this to the house, are you sure you can have a front loading garage?

If you have to have a side loading garage is the lot wide enough?

Are you willing to obscure your own view of the lake?

You would also have to have it checked out to make sure your foundation will even support the weight of an upper addition.

Thanks, everyone!

The new structure will have its own foundation and shouldnt block anyones view. It will, however, add views for the office and workout rooms we were going to put up there.

What about something like this? I cut and paste within the context of a two stall plan

with a one bedroom apartment along the back. The apartment was a bit convoluted, but left the entry bath and storage as is. I am assuming this was to give visitor access to the bathroom without going through the bedroom. And the kitchen was on the exterior back which buried the LR in the middle so I juggled that.

I also think this is two small for a two car garage, I would want it several feet wider and deeper.

I feel like this sort of reiterates your house, this also could be mostly glass along the lake view

Thanks, Pal! I don’t think our lot is wide enough to put the garage doors on the side, they will have to face the street. I’m sorry I missed that question above! honestly the desire to have another lake view from the ”apartment” which may not be a rental for quite some time and would probably be the best selling point as a rental, feels like a must. We agree with the poster above that the stairs should be enclosed.

The stairs would probably go up where the bathroom is and kitchen and bath would go to that wall the bedroom and living room are on. Thoughts?

Do you need it to be two story at all? The above has the garage doors facing the street, or the back of your house, and the LR and BR facing the lake view and could be all glass. And it would not be taller than your house.

Sometimes it's best to stop monkeying around with a very small, older house and start over with a teardown/new build -- especially if you're doubling the size of the place. Or...sell and move.

Our DS once had 18 old cars on the lot of the small house he owned at age 24 -- and shared with a friend/mechanic. After he married, he sold that house (and Auto Wreckers removed the hulks). He used a cement pad at the home he and DW bought to work on ONE car. That lasted until Baby One. The hobby car was sold. The pad most recently housed an above-ground pool for the kids. Things change!