Big buildings filled with rental storage lockers should keep rising on O'ahu over the next few years, and the next two self-storage facilities are planned in Mililani and Kapolei by local developer MW Group.

MW yesterday announced plans to build two facilities it expects to open in spring 2008, adding another roughly 230,000 square feet of storage space to the rapidly expanding market.

The company expects to start construction in the next few weeks on Hawaii Self Storage-Mililani at Wika'o and Akamainui streets. The mostly three-story building will contain 109,950 square feet of storage space.

Construction of Hawaii Self Storage-Kapolei is expected to start later this month or in June on Kalaeloa Boulevard between Home Depot and Big Kmart. The three-story building will contain 113,400 square feet of storage space.

The two projects will join 40 existing self-storage facilities on O'ahu that provide almost 2.4 million square feet of space, but there's still room for more based on projected demand, according to local real-estate appraisal and consulting firm Lesher Chee Stadlbauer Inc.

Stephen Stadlbauer, a principal of the consulting firm, said U.S. industry data suggest that Hawai'i is the most underserved market in the nation with 2.7 square feet of storage space per resident.

The firm estimates the market can handle 4.7 square feet to 5.7 square feet per person. Stadlbauer said there are at least 17 planned self-storage facilities in Hawai'i that would deliver roughly 1 million square feet of new space to raise the per-person equivalent to about 3.9 square feet.

But Stadlbauer also said overbuilding could result in the next two to four years.

Trying to capitalize on the projected demand, local and Mainland self-storage developers have added about a dozen facilities on O'ahu in the past six years, including one on waterfront property in Hawai'i Kai.

Recently, a strip mall was demolished to make way for The Lock Up, an eight-story storage building on the makai-'ewa corner of McCully Street and Kapi'olani Boulevard.

Last month, another developer opened the 31,000-square-foot Simply Storage of Hawaii, a facility created by renovating the former Bakery Europa warehouse in Iwilei's Cannery Row.

"There are just so many facilities coming up," said Simply Storage developer Todd Middleton. "It's quite astounding."

Developers expect much demand from the construction of several residential condominium towers, which typically come with limited storage space, as well as from users of older self-storage facilities that lack modern amenities such as climate control, security and 24-hour access.

MW said it plans to operate eight facilities on O'ahu by 2008.

"We have long realized that there is a great need for modern storage facilities in Hawai'i due to smaller homes, limited garage space and no attics or basements," said Mike Wood, an MW principal.

Reach Andrew Gomes at agomes@honoluluadvertiser.com.