There is a very entertaining old movie with Cary Grant and Myrna Loy that I highly recommend. “Mr. Blandings Builds His Dream House’’ (1948)shows the pitfalls (and joys) of home construction in a comical way.

In one of my favorite scenes, Mr. and Mrs. Blandings try to define what they really need for bathrooms in their new house:

Muriel Blandings: “I refuse to endanger the lives of my children in a house with less than four bathrooms.’’

Jim Blandings: “For thirteen hundred dollars they can live in a house with three bathrooms and rough it.’’

A lot of decisions go into designing a bathroom. When it comes to master bath design, a common question is whether you need to have a separate tub and shower to boost your home’s resale value. If you have the space, why not do both? If not, the trend is to do a good-sized shower, perhaps one large enough for two people. This works well if you have a tub in another bathroom.

The first thing your plumber will need is the shower valve, a behind-the-scenes item that is specific to a brand and a type (pressure-balanced or thermostatic). Pressure-balanced valves allow you to control only the temperature, whereas thermostatic ones give you command over the flow, too. Having the valve on hand will buy you a little time to figure out your look. Ideally, though, you should have everything picked out before the plumber digs in.

Also, do you want a hand-held shower head, a fixed one, or both? Do you want body sprays? Have you always wanted a rain-shower head? I am a big fan of having a hand-held shower head on a bar that you can adjust in height. It is personal choice, so think about what is important to you.

One feature of a master bath that is desirable if space allows is a water closet, a toilet in its own little room. It offers more privacy if two people are using the bathroom at the same time.

Some clients become overwhelmed when it comes to defining the “look.’’ First, go shopping: Find something that grabs you, something that will make you happy every time you see it — whether it’s a tile or a countertop material or a fixture. Once you have identified one element you love, it will be easier to find coordinating elements. Your architect or interior designer can help you with this.

Lighting is also key. What kind of wall sconces, if any, do you want? Will you have the space to place them on either side of the medicine cabinet or mirror over the vanity? Or do you want a single one over the vanity? Side lighting is preferable, but sometimes there isn’t the space. One solution is to install medicine cabinets with built-in lights along the sides.

If you are using tile, installing an electric mat under the floor is another nice feature. It often comes with a thermostat or a timer. The mat is not a source of heat for the room, but it does warm up the tile itself, which feels wonderful underfoot in winter.

Last but not least, be sure to provide enough natural light in the bathroom. If possible, align the window with the door so that when the latter is open, natural light can filter into the bedroom. This also provides additional visual access to the outdoors from the bedroom.

Chris Chu is an architect in West Newton who specializes in residential design. Send questions to Address@globe.com.