Government guidelines say the maximum time for a low-priority case to be dealt with should be 52 weeks.
But a snapshot survey of 30 recent cases in Hull has revealed a third failed to hit that target.
The longest took just over 70 weeks from the start of the assessment process to the work being finished.
Typically, adaptations range from grab-rails and stairlifts to ramps and disabled toilet facilities.
Mark Baggley, chairman of the Hull Choices and Rights Disability Coalition, said: “I am not surprised by the figures because it really is a massive problem which, unfortunately, isn't really known about.
“I know of one couple who have been together 50 years, but because one of them has been in hospital and their home requires adaptations, they have lived apart for the past eight months because the house had yet to be adapted.”
Mr Baggley said a shortage of occupational therapists and building surveyors were among the reasons behind the delays.
But he added: “The biggest factor is that most of the city's housing stock is just not suitable.”
John Hocking, the council's housing director, who deals with disabled adaptations, said: “We have a fast-track system for people in greatest need, but we do need to improve the time it takes.”