OK LTF365 I know you've been anxiously waiting for my input!
You will find a home designer to be cheaper then an architect. You don't have to have architect drawn plans to build a house. Also don't buy plans online or out of a magazine. They have to be adapted to work here so all it does is add to the cost. I bought a '$125 design your own home' software and did my own. Mine is just a box though so it was pretty easy. I gave that to the builder who tweaked it a little then took it to the county where the building department guys helped with the engineering. After a trip to Kinkos we submitted it back to the county with a large check and we were done.

If you're adding 4' to the front I'd put a lot of that space into your utility room. Your plans don't show a water heater or furnace? I went with a heat pump and my evaporator is in the attic. They did some fancy trusses to make a little room upstairs for it. I wish I'd known and I would have had them run the same style trusses all the way to the end so I could store more bodies.

I went with 3' foundation walls and hung the floor joists. Doing this lowers the inside floor of the house to reduce or eliminate steps. I have a 5" step up into the house. I didn't want it level because of rain and snow and such.

If your back porch is on the weather side of your house consider bringing it out further. I did mine 22' x 24' and the patio table is always dry and in the shade. I plan to do an outdoor kitchen in the future so there is a capped off drain and conduit to run water lines out there already.

Speaking of conduit, I ran 2" conduit from two walls in the living room, one in the dining, and one from the back porch to a central location in the attic. Also meeting up there is 3/4" conduit that runs to blank wall plates in the bedrooms and other places around the house. Then there is another 2" that runs from the crawl space to the attic. Not knowing what the future holds if I ever need to run speaker wires, tv, phone, fiber optic, etc it'll be a breeze.

The cost to go with wider interior doors is minimal and a good place for a builder to skimp. Get doors that you can move a washer, dryer and other furnishings through with ease. The same goes for windows, a little bigger isn't that much more. My neighbor did a Hiline home and these were the first things I noticed, skinny windows and doors.

Just before insulation photograph every wall. This makes it easier when you go to add something in the future and know what's in there. Before back fill measure where everything goes in and out of the house and write it down.

Insulating certain interior walls will be a big plus where you want to help with noise. Since your moving your guest bath next to the living room I'd do that one for sure as well as the master bedroom.

If your heat is NG or LPG consider running a hard line to the patio for your grill. I did mine now I never have to deal with running out.

A hot water spigot to the outside where you wash your cars and boat is a must.

I went with a big walk in shower like you were looking at. I love it. It even has a little bench so you can sit down if you get tired of taking a shower! The shower was one of the things on my must have non negotiable list.

Consider running double or triple duplex' in areas that may get full.

If your going to do certain parts of the build yourself be realistic on your abilities and time. Even something as easy as painting, you have to be able and ready when it's time. There will be people waiting on you, trim carpenters, electrical trim etc. Even though I would have loved to do it all or even more myself on mine time was my biggest factor. The job that pays for the house has priority so I only opted to do the day to day clean up and the final cleaning. I also did the conduit and interior insulation mentioned above and out of pocket. Even with only doing that I was busy every weekend.

There will be a lot of ups and downs through the whole process but once you're done you'll want to do it again. I'll stfu now.

Edited to add more babbling!
It's cheaper to build in the summer than in the winter so no need to rush. Also, if you're the festive type, you may want to add Christmas light outlets in the upper outside corners. OK now I'll stfu.