Could you post the entire floor plan?

Do you need a full sized bathroom there or a closet in the office? If not stealing the space from those two areas could make the mudroom bigger.

The room is so small it doesn't seem very useful for anything and the orientation of the door swing will limit your wall space even more.

Tell your architect to stop pricing and redesign until it is all the way you want it.

The bathroom layout isn't great, either. Very very small shower and one would have to walk around the toilet to get to it...

I'm thinking about converting the walk-in-pantry into a regular shelf to save the space. Then I can expand the mudroom. Here is the entire floor plan. I would like to keep the office closet as we are going to use it as a bed room soon and need a full bath there. If I could find another place for the closet opening from foyer into the office room, then I can use the foyer closet space for the office room and use the office room closet space to expand mudroom. Any suggestions where I can get a closet space mainly for guest coats?

Without seeing the full plan or at least what the adjoining rooms look like its hard to give advise however based on what I see and to keep the changes minimal I would push out the wall with the door into the living room to. line up with the pantry door wall. Then push the door from the kitchen closer to what I assume is the living room and make it a sliding door.

This should give you room for a bench and hooks along the pantry wall I'd put a built in closet for hanging coats along the closet wall.

Still not the biggest mudroom but better than it currently is.

IMO that is not a walk in pantry get akitchen designer to help architects are very poor KDs in my experience. Nobody puts a desk in a kitchen anymore we all have laptops that go where ever we want.That mud room is useless unless there are only 2 of you .Use the space of the pantry to make the mud room a proper size design pullout pantries on the wall in the kitchen tha adjoins the mud room you will get 2x as much storage as you get with that pantry .A range should always be on a n outside wall if possible and you have the space for that you better get some roller skates to cook in that space. GET A KD NOW

"architects are very poor KDs in my experience."

Patricia never met me.

Look at options for coat storage beneath the stairs.

Re-assess the need for three eating areas.

Envision entering the house. As a walkway and stairs, there is room for improvement.

The kitchen is in need of work. For one, obstructing the main kitchen walkways is undesirable. For another, separating the kitchen walkways from other circulation is desirable.
If you can do that, and move the pantry to whatever extent, you can work on the mudroom.

"You" meaning your architect, who hopefully has more knowledge and immagination in this regard than I do. Good luck

Lots of good options here. Do you really need a door to your mudroom? We have just a framed opening. At a minimum, I would make that door a pocket door as it will likely be often open.