I would double check your assumptions about permitting and taxes. The places with which I am familiar require permits and inspections for that kind of addition, especially a bathroom.

If you are on septic, are you allowed to add a bed and bath? Your local building department should be able to help you with codes in your area.

The kind of addition you are talking about will not be cheap: foundation, roof, siding, windows, plumbing, electrical, HVAC - for starters. Others will know better, but this has to cost at least 100k. How much more can the additional planning fees be in comparison?

Taxes are usually based on your home's value, so whether you add 499 or 500 sq. ft. won't matter. Do the addition that works for you.

I know it’s not cheap and I know a permit is involved no matter what but the cost and approval process and length of time it takes is much greater in Southern California when it exceeds 500 square feet.

400 sq ft seems like plenty to make a nice space. Can you show a small sketch of the 499 sq ft outline, where it attaches to the house, which direction it faces, and what the views are outside the potential windows?

Also, do you want the office to have its own door and walled in area, or can it be more open to the space?

Do you want a king or queen? Will you have dressers in the bedroom or in the walk in closet?

"Seems like plenty" ...........and that DEPENDS

Where do you enter

What constitutes "office"

What does a walk in closet mean to you?

What constitutes a tub, and under no circumstances will you get BIG everything in a bath- I wouldn't plan free standing.

No office..... below as WHO KNOWS? Not us.

Get an architect to your site. Give him your wish list. Anything worth doing, is worth doing right. You need the drawings, the permits et al. There are no great short cuts to a good result

Way to much missing info to help. At the very least we need a to scale floor plan of the space as it is planned with all windows doorways and info post here in comments . Post the plan in jpeg format show us where the plumbing has to go . All of that here DO NOT start another post.

It isn't "planned" : )

That's the question.....lol

Be careful not to be a penny wise and pound foolish. Design an addition to meet your needs and fit your site, then evaluate "the permitting and costs fees and taxes". You may find going over the 500 square feet may benefit you more that paying a little more.

Jan's pic is an excellent place for you to start brainstorming. Put a desk on the entry door wall and you have office space. Does this satisfy what you want in closet, bath, room size? What does or doesn't work? Do you still want to stick to 499 sq ft or is it now worth it to go bigger?

Most importantly, do you have a realistic budget for this project? What percentage of the budget is the higher cost, fees, and taxes? Could you easily go over budget on the small addition and wish you'd just waited longer, saved more, and had the really right space?