Determining what you need

Begin by deciding whether you really need plumbing at all. If you plan to use the outbuilding for a home office, woodworking shop, or getaway, plumbing may not be necessary. However, be aware that adding plumbing when walls and floors are completed will be about twice as expensive as adding plumbing from the beginning of your project.
 
Supply and drain lines must be installed underground, and drain lines may prove challenging to link to a municipal sewer or septic system. Plan on spending about $3,000 to establish basic service.

You’ll add about $2,000 if you plan to install a toilet, bathroom sink, and utility sink—about $5,000 total. Here are some specific items you may need, depending on the use of your structure:

  • Woodworking shop: Clean-up utility sink
  • Ceramics or painter’s studio: Clean-up utility sink
  • Car restoration or machine shop: Clean-up utility sink with an on-demand water heater, hose bib
  • Retreat: Full bath with whirlpool tub, bar sink
  • Small business headquarters: Bar sink for snack area, second water meter to keep utility costs separate

Be sure to brainstorm possible future uses of the structure. For example, should you sell your home, the new owners may be more interested in a guest house than a stained-glass art studio. That argues for laying the groundwork for a full bath.

“I wouldn’t build such a structure without getting water out there and planning on a drain if it is at all feasible,” asserts Roger Peugeot, a plumbing contractor in Overland Park, Kan. Roughing in lines for a full bathroom in the future would cost less than $200. “You are talking about adding a few feet of pipe under the slab. It would be really insignificant.”

Jeff Hunt, a contractor in Houston, agrees with the wisdom of roughing in for future needs: “You’d be crazy not to stub out bathroom supply and drain lines for future use, and stub gas lines where you might need them.” And think accessibility, he adds. Frame for grab bars and a 3-foot wide bathroom door just in case an elderly parent someday uses the building.

Supply lines and drains

Supply lines are relatively straightforward to install. The bulk of the time and expense comes from digging a trench from your home to the outbuilding location.

Drain lines, however, have to flow toward the sewer or septic tank. They must be set at an incline of at least ¼-inch per foot for gravity to do its job. Avoid locating your studio on ground that is lower than the sewer line or septic tank.

If you’re workshop or studio is located on the low end of a slope, you may have to install a waste catchment and an ejector pump to move waste water up to a sewer line of septic tank. If you must build on a location where the sewage has to be pumped, costs of installing underground supply and drain pipe jump to about $5,000. Even if you’re on the high end of a slope, you may have to add some turns and zigzags so solid waste is carried away with the liquid.

“A lot of this can be overcome when you are designing the building and locating where it is going to be,” says Peugeot. “Locate it so that drain can be run with gravity so you don’t have to pump it. If you had a big lot it would be better to have the building on the side of the house rather in back of the lot.”

Adding a water heater

Household hot water systems accommodate lots of demand, often simultaneously supplying hot water to sinks, showers, and a dishwasher or washing machine. An outbuilding makes much more modest demands, and you’ll need hot water only when you’re in the building, likely only a small portion of the day.

With that in mind, consider installing a 6-gallon electric water heater that can be switched on when you enter the outbuilding, delivering hot water within 30 minutes. Installed cost is about $400.

Another alternative is a low-capacity instantaneous heater. “They produce about a half gallon of water per minute,” says Peugeot. “They’re about the size of a milk jug and cost about $200.” While a faucet typically produces a heftier 1½ gallons per minute, the compactness and low cost of low-capacity units make them a great alternative. Assume an installation cost of about $200.

Assessing the add-ons

Beyond the basics of a toilet, bathroom sink, and utility sink, your new outbuilding may call for some additional features. Here are the installed costs you can expect:

  • Shower: $800–$1,100
  • Whirlpool bath: $1,100–$3,000 depending on the elaborateness of the tub and another $2,000 for an on-demand water heater
  • Gas line: $800 when added to an existing trench
  • Bar sink: $250
  • Water meter: $300 on up, depending on the municipality

Avoiding frozen pipes

“To prevent frozen pipes, bring the water line up into the middle of the structure,” Peugeot recommends. “That’s where it is always going to be warm. Never put any pipes on an outside wall in a cold climate.”

Of course, even a cozy central location won’t do the job if there isn’t some consistent minimal heat. A little warmth does more than just keeping the pipes from freezing. “To keep the paint and the sheetrock in good shape and just keep the building in good order, I wouldn’t recommend letting it go below 50 degrees,” suggests Peugeot. Four 1500-watt electric baseboard heaters fit the bill for a 600-square-foot building.

If you expect to shut down your outbuilding for the winter, locate a shutoff valve inside your basement. If you have natural slope in the line from your house to your studio, a drain-down valve will completely remove water from the system. Otherwise, you’ll need to blow out the pipes with compressed air.