Why are utility bills so high in my in-law suite?

A high bill in an in-law suite almost always traces back to its heating and cooling, not its lights or appliances. The usual causes are leaky or badly placed ductwork, expensive electric resistance heat, equipment that is the wrong size for a small space, and air leaking around the part of the building that was added or converted. A licensed HVAC contractor can tell you which one applies.

Start with the ducts

If the suite shares the main house's forced-air system, the ducts are the first suspect. ENERGY STAR says that in a typical house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.^[1] Suites are often at the far end of a long duct run, and ENERGY STAR lists ducts located in an attic, crawlspace, or the garage as a sign of poorly performing ducts.^[1] A garage conversion or an addition over a crawlspace often fits that description.

A contractor can test duct leakage and seal and insulate what is accessible. If the main system was sized for the original house, it may also be stretched thin by the extra floor area.

Electric resistance heat

Many suites get baseboard heaters, wall heaters or an electric furnace because they are cheap to install. They are also expensive to run. ENERGY STAR describes mini splits as a fit for homes with costly electric heat (e.g., baseboard, furnace, wall heaters, electric radiant) that will also benefit from cooling.^[2] It also names additions or outbuildings (e.g., shed, barn, garage) where extending ductwork or heating/cooling capacity is difficult as a common use.^[2] If your suite is on baseboard heat, a ductless heat pump is the usual comparison to price.

Equipment sized wrong

Bigger is not better for a small space. Virginia Cooperative Extension warns that an oversized air conditioner will "short cycle."^[3] It cools too quickly to remove moisture well, and the suite feels cool and clammy. The typical response is to turn the thermostat down, and every degree the thermostat is lowered increases cooling bills 3% to 5%.^[3] A system that is too small runs nonstop and still falls behind.

The fix is a proper calculation. Extension says load calculations are usually performed for each zone in a dwelling in accordance with ACCA Manual J, and that an accurate load analysis will help to prevent problems that occur when a house is equipped with an oversized, or undersized, system.^[3] Ask for that calculation for the suite itself, not a guess based on the old equipment.

Air leaks and insulation

Conversions and additions leak where new meets old: rim joists, attic or ceiling connections, around windows and doors, and chases for pipes and wires. ENERGY STAR lists high heating or cooling bills, drafty rooms and hot and cold rooms among the common symptoms of attic air leaks.^[4] A basement or garage suite can have the same problem with uninsulated slab edges, walls and ceilings. A contractor with a blower door test can find the leaks and tell you what to seal and insulate first.

Check how the suite is served

Some bills are not a fault at all. Ask these questions before paying for repairs:

• Does the suite have its own meter, or is it on the main house's bill?
• Is the water heater shared, and how old is it?
• Is a space heater, a second refrigerator or a dehumidifier running full time?
• Are the thermostat schedule and settings what you think they are?

If the suite is rented or lived in by family, a shared meter can hide real use and cause friction. A separate meter is something to discuss with your utility and a licensed electrician; see what building an in-law suite involves for where it fits in the design. What the original build cost, and where an energy-efficient design adds to that, is in how much an ADU costs in Virginia.

When to call a licensed pro

For the diagnosis and every fix above. Duct sealing, a new load calculation, a mini split, a new circuit or a meter change are all licensed work, and some need a permit. Virginia's contractor licence classes are set partly by contract value: Class C for a single contract over $1,000 but less than $30,000, Class B for $30,000 or more but less than $150,000, and Class A for $150,000 or more.^[5] Under the same law, the Board shall require a master tradesmen license as a condition of licensure for electrical, plumbing, and heating, ventilation, and air conditioning contractors.^[5] Check the licence before you sign.

Ask each contractor to put in writing what they tested, what they found and what they propose, so quotes can be compared. If the suite was built recently, ask the original builder too, since a sizing or duct problem may be a defect. Also see whether an accessory dwelling unit is allowed where you live before making major changes.

Browse remodeling services or read more remodeling answers. To get prices for your suite, describe the job once and licensed local HVAC businesses quote it.