How big is the planned house? Double the size. That ought to give you a good sized sewing room!:D

Seriously, it depends on how much 'stuff' you have accumulated over the years, how much of it you plan to keep and how much you plan to add.

If you can swing it, ask for a 15'x15' room. (I would love a sewing room that is minimum of 20'x20', which is a little smaller than a roomy 2 car garage.) That should give you most of the room you need. However, no matter how much room you actually have, it is never enough.

Also, would the long arm be on a frame or on a table? That makes a difference. Long arm takes up a lot more room, even a 'small' one.

Mine, too, but it would also have to have at least 6 garages for my husband and his stuff.

I don't know what would work for you, but my sewing room sits over the two-car garage. It holds my longarm, one sewing machine, a big cutting table, and tons of fabric. I can function in it very nicely, yet I wish it were bigger. When I was moving and looking for a house, I had to pass up a number of homes because they didn't have a space for the longarm. I wound up with a home that was bigger than I needed, only because the smaller homes didn't have a separate room large enough for the longarm. I envy your ability to design and build your own home with the sewing room of your dreams.

Correction needed: My husband's shop is 24' x 36'.

I have two sewing rooms both are about 12 x 14.

One has my cutting table, sewing machine, embroidery machine and ironing board along with the bulk of my fabric.

The second one has my sit down longarm machine along with some odds and ends.

one can "make do" with a small space, but go as large as you can.

Ask for 60x80 and let us know what he says🤣

I would hazard a guess that it depends on what/how much you're planning on putting in it, if for instance a long arm on a frame, takes up quite a bit of room. My sewing room is a separate building and it's 14x32 and it's NOT big enough. LOL

Chasing Hawk- that's my dream room also! :D
Sara- I don't have any advice but it's so exciting that you can design your room! I hope you will keep us posted and good luck :)

Now THERE'S a house plan with all the priorities straight!

No matter what size you build, you'll wish you had it bigger. Space has a tendency to fill up. I had to have 5 storage racks just for my stash. I also have an "ironing station" which is a changing table with a tall cabinet on one side and three large drawers.