OldRoads wrote:

I've talked to the local zoning board and it
looks like I can legally convert the attached
garage and storage area on my rental property
into a one bedroom in-law apartment.

Be very careful when dealing with Zoning Boards.
What you are creating is known as a "Coach House."
In many locales, coach houses are NOT rentable as
separate units. However, I read in another post
that you wanted to use it as an owner's apartment.
That I suspect is permittable.

My next step is to get an idea of what it would cost.
The garage has no plumbing at all.
I'm just making wild guesses, but I'm thinking $5k
to hook into the house's water & sewer, $5k for a
1/2 bathroom, $10k for a small kitchen, $5k for
wiring and electric heat.

Really, I'm just pulling these numbers out of thin air.
Has anyone here done this type of conversion?

My thin-air opinion is that your costs are low.
The sewer hook up may require breaking up a concrete
a concrete floor so I'd guess 8 to 10K for the
the sewer/water hook ups.

You need a shower in the 1/2 bath so I'd put that
at 7 to 8K.

10K for a small kitchen might be high. But 5k for
the wiring and electrical heat is probably low.

OTOH if you have the ability to act as your own
general contractor, your costs may be lower.

It will come as a shock to me if they let you put
in separate meters for the electricty.

Dick