Now even a modest master bath and good size walk in closet may require 200 square feet to accommodate two sinks, a tub, separate shower and toilet compartment. His and her closets are often the first feature that is desired that will add footage. Some homeowners consider a tub unnecessary and wasteful potentially saving some space but that footage is often used elsewhere in the bath, particularly in the shower.
On a luxury home there may be 200 square feet of master closets alone. With amenities like built in chests, dedicated shelves for shoes and purses, hanging area for belts and ties and mirrored dressing areas that is possible. I have even designed coffee bars with a bar sink in dressing areas. These master baths including closets may be somewhere between 400 and 500 square feet.
There usually is an association between the master bath and house square footage of 10%. A 200 square foot master bath would be appropriate in a 2000 square foot house. This is a good design rule to follow but ultimately it is determined by what features you desire although resale should be considered. By today's standards the modest master bath described should be the minimum.
Things to consider that will increase the modest bath size are separate vanities, a makeup knee space at the vanity, large tub decks and door less showers. Flat screen wall mounted televisions have made this feature more convenient. There are also mirrors with built in televisions that look just like any other mirror until the television is turned on. A luxury sized bath should always have his and her closets and may even have his and her toilet compartments.
Master bath remodels within rectangular spaces usually work the best. Square spaces tend to have wasted space in the center but a little creativity can overcome this. If the space is large enough it might be perfect for an island tub or claw foot tub. Or a shower may jut out from the wall with glass on three sides.
It is certainly determined by you how modest or luxurious the master bath is. Through creativity though even modest baths can be distinctive. We have come a long way as master baths although private have evolved to compete with the kitchen as the showcase of the modern home.