When Alec and Jennifer Harmes spent 4,900 for their first home in 2011—a 1,500-square-foot ranch fixer-­upper in Austin, Texas—they assumed they would be living there for many years.

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They can house an additional family member or provide rental income—­allowing baby boomers to afford their house once they retire or helping millennials pay the mortgage.

More municipalities, particularly in Western cities, are amending zoning laws to allow for ADUs. Younger buyers in particular say they want a dedicated laundry room, perhaps off the kitchen or even near second-floor bedrooms.

Buyers of all kinds have long focused on the kitchen, but it holds particular sway over the newest wave of first-time homeowners.

A “modern/updated kitchen” topped the list of ideal home features in our survey of millennials, registering as most important to more than a third of respondents.

Though the Harmeses saved big by doing most of the work themselves (he works in construction management, she oversaw design), the total investment was close to $65,000.

They were even planning to build a separate mother-in-law apartment on the property to help lure family to Austin.

They refinished the kitchen cabinets, and installed new stainless-steel appliances and LED lighting.

New engineered wood floors replaced the mishmash of linoleum tiles and musty, high-maintenance carpeting.

But their folks didn’t want to relocate, so they made the tough decision to move back to Florida to be close to them.

“If we could have picked up that house and brought it with us, we would have,” Jennifer says.

But it’s also a testament to the couple’s savvy instincts about what today’s buyers are looking for, especially now that millennials, 75 million strong, have become the leading cohort of buyers, purchasing 32 percent of homes in 2014.