If the two rooms share a wall that might move, yes — designing them together avoids a scenario where the finished bathroom leaves the closet layout compromised, and it's typically more efficient than two separate projects months apart.

There's no strict minimum beyond standard fixture clearances, but most primary suites that feel comfortable for two people run 100+ square feet, with dual vanities, a separate shower, and often a soaking tub or separated water closet.

It depends on where the existing plumbing runs. Adding a second bowl next to the existing vanity plumbing is straightforward; putting the second sink on an opposite wall means new supply and drain lines, which is a bigger scope.

It's a preference, not a requirement — plenty of well-designed primary suites keep the toilet in the open room, often with a partial wall for visual privacy instead of a full enclosed room with its own door.

Longer, mainly due to size and complexity — where a hall bath might run four to six weeks, a full primary suite remodel with layout changes often runs six to ten weeks, more if it extends into the closet.

Scale (more tile, more cabinetry, more fixtures) and complexity (dual plumbing runs, possibly a separated water closet) both add cost beyond what the size difference alone would suggest.

Yes — keeping fixtures on their existing plumbing walls avoids slab cutting entirely, which is one of the more effective ways to control cost on a primary suite remodel without compromising the finish quality.

Not planning storage (linen, towels, toiletries) as its own design category — it tends to get treated as an afterthought once the vanity and shower are decided, then feels tight once the room is finished.