Scarborough Borough Council's planning committee unanimously approved the proposals yesterday for Kepwick House which was built 14 years ago as part of the Sands development.
An agent for the applicants Benchmark told a meeting that the penthouse structures would be lifted onto the building by crane during a 12-week period in winter in order to minimise disruption during the tourism season.
However, residents have expressed concerns that this could take much longer and become a "blight" on the North Bay.
Colin Hale, who lives at Kepwick House and works as a construction surveyor, told the meeting: "Believe me this would take more than 12 weeks – there will be at least a year to two years of disruption.
"Does the council really want to give up on two years of tourism in the area?"
There were also complaints over the appearance of the proposals, with ward councillor Guy Smith saying the penthouses will "destroy" views of the area
A total of 50 objections were made against the plans, including from several Kepwick House residents who raised further concerns over the site's "inadequate" parking.
This comes after permission for three apartments on the roof was previously granted on appeal in 2015, however, these plans never came forward.
The latest proposals – which include nine new parking spaces – were recommended for approval by council officers which said the design of the extension was considered to be "acceptable".
This was a view shared by councillor Will Forbes who said the development would "match" the adjacent Lockton House which already has penthouses.
Councillor Bill Chat was also in support of the proposals. He added: "I remember the feelings people had in Scarborough when we said we were going to build the Sands complex.
"I for one was against it because I thought it was a big monstrosity. But as time has gone, it has become acceptable and people see it as a good development.
"It has enhanced that side of Scarborough.
"And I don't honestly see how creating nine more accommodation units is going to affect anything more than creating nine more places where people can come on holiday.
"Development is something we must do. If we don't develop, we become stagnant."
Councillors cast nine votes for and none against to approve the plans at yesterday's meeting.