The board usually votes after hearing all requests, which was about midnight Wednesday at the end of the roughly five-hour meeting, planning officials said.
"We had no idea it would be approved. I'm very disappointed," said Bernard C. Zimmerman, one of several neighbors to attend the meeting and leave before the late vote was cast.
Zimmerman, of Fairchild Avenue, said the vote "stinks" and was "ridiculous."
Zimmerman said he was concerned the center, which could accommodate 103 residents, would cause traffic and noise problems.
"I don't think too much of the decision at all. I think they made a bad decision," said James B. Robinson, whose Beechwood Drive home borders the back of the 4.62-acre site at 1201 Pennsylvania Ave.
Robinson said he was concerned nearby homes would lose privacy because several trees between the residential area and Pennsylvania Avenue will be torn down.
He also said he was concerned about stormwater drainage and traffic problems.
"I'm very upset," said Dot McDonald, of Fairchild Avenue.
Residents have enough problems getting onto busy U.S. 11, she said.
McDonald, a realtor, also said she was concerned property values would go down.
Board member Tony Dattilio said a 50-foot buffer zone will remain between the center and nearby residences. Some of the trees will remain standing, he said.
"It looks like a beautiful facility that's going to be built," Dattilio said.
Since the center's expected 50 employees will be on different shifts, Dattilio said he didn't expect major traffic problems.
Manorhouse officials could not be reached for comment on Friday.
Construction is expected to begin in the spring and be done in a year, officials had said.
The board also unanimously approved a request for a special exception for an assisted living group home for about eight people by the Washington County Association for Retarded Citizens at 343 S. Potomac St., Davis said.