Maybe you could just ask your husband to build a one story house with all the normal rooms....LR, kitchen, bedrooms baths etc and to add a one room second floor to fit the footprint of the main floor for your sewing room. Then you would really have a great size room depending on how much square footage the main house has. Oh and be sure to include an elevator to make it easier to access. A large elevator! and a bathroom of course.
You did say dream sewing room!! That would be mine.

As big as possible. It will fill remarkably fast.

My thoughts exactlly.

If you are going to have a basement, I would make it the length and width of the house, finish it and use that as a sewing room.

If you can get measurements of all you plan on having in there might even double it and lay it out on graph paper. Then plan from there. My sewing room is 32 by 18 and it isn't big enough. The long arm takes up half of that and I have 4 other sewing machines set up in there. there is no room for a big layout/cutting table. It started out being the master and bath but I am single so don't need that much room in a bedroom so made it all one room for sewing. couldn't go any bigger wish I could have. Since you do have a chance to get what you will need always go bigger. Remember to take into count your movement around the room and machines while you are sewing.
Lynda

What a wonderful project!

I don't know what size room you would need, but you have some great advice here.

I think it's a blessing to be able to sew with your mom. With that in mind, you might consider a multi-purpose sewing table for her where you could lower that machine and have a flat work surface when she's not sewing with you.

Have a blast planning this one!

LMBO, my dream house too, except I'm not sure I really want to waste space on that huge kitchen. ;)

Yes, this is the same size “dream sewing area” I showed my husband. I also want the bathroom to be big enough that I do not have to straddle the toilet to close the door......perhaps I should just ask that the door swing out so I will have more sewing space......

Put in a sliding door or hang a curtain in the doorway.

We opted for folding shutter style doors. They work great! Put them on the closet too. Very pretty frosted glass panels.