Yes, there are definitely options not too far from Boston but you might pay the price depending on location. Any idea what your budget might be? Also 20 minutes outside of Boston limits you to very dense areas (smaller yards that don't allow for second dwellings .. again, price permitting)
Here are some options that I chose by hand. I had to search using all sorts of key words since the MLS system that agents use to advertise their listings do not have criteria for second dwellings.There are a couple interesting options here all chosen without knowing your budget.
1) The Brookline house if you want a more city-like feel: It has an attached garage that you could perhaps convert into a separate space and use as your studio. The price is amazing for where it's located but looks like it needs some renovating; 2) The Weston house w/ a carriage house on the property for affordable price tag (for Weston at least), downside it's located on a busy street; 3) Gorgeous home in Belmont with a carriage house on property that needs to be finished. Higher price tag but has been on the market for a while so perhaps you could get a good deal. It is also currently configured as a 2 family but maybe that would be a good thing for a while so you could rent out one aptmnt to help with the higher mortgage payment and convert to a single later; 4) Stunning Greek revival on busy street in Lexington that has a carriage house also available for sep purchase; 5) I give this one 5 stars but again you will have to pay the price: Amazing Winchester home with tons of architectural flare and a 1500 sqre ft carriage house; 6) High price tag but gorgeous home in Wellesley with barn; 7) Framingham, a little farther commute but not bad, plus price is right and it has carriage house (busy road); 8) Roslindale colonial with barn/garage that needs reno's, great commute; and 9) Another Weston home on Corwood drive, 3 acres plus a barn/shed, higher price.
There is also one other place in Wellesley (00 (&402) Worcester Street), busy street, nice price with a barn. With your criteria you will need to sacrifice something so just keep that in mind.
Here is the link to all the listings:
Concord and Carlisle are also gorgeous places to live and there are some options out there with barns/carriage houses:
Another great opp in Melrose but it just went Under Agreement. You never know the deal could fall apart. The house needs updating:
Like you mentioned, another great option would be a house with land to build your own carriage house/studio. Just remember you will need to do more upfront due diligence before purchase to confirm it's a build-able lot.
I hope that helps!
Terriory.com, A Destination for Home Buyer's