The work carried out will involve widening doorways, installing lifts and ramps and other alterations to assist access.
Economic forecaster Martin Hewes predicted that the total bill for compliance would be up to £100m a year.
It is understood that leading finance and property firms are preparing to meet the demands of the legislation. A spokesperson for insurance group Prudential said its property arm, Prudential Property Investment Managers, is investigating the implications of the legislation and has undertaken a study of a sample of its buildings to assess the implications for the portfolio as a whole.
She said: "We are still analysing the findings of this study, and cannot, therefore, give any indication of the measures that might be necessary, or the likely overall costs at this stage."
Sudden upturns in demand can lead to a boom-and-bust cycle which destabilises the industry as a whole
Barry Stephens, National Federation of Builders
A UK-wide survey of 300 small and medium-sized businesses, carried out by law firm Pennington's, concluded that most firms are poorly prepared for the new legislation.
The survey reported that only 20% of respondents described themselves as being "very aware" of the act; 52% had a limited understanding.
Barry Stephens, director of policy and planning at the National Federation of Builders, said there would be an increase in demand for medium-sized contractors and builders.
He said the key concern is to ensure that the industry grows at a steady and sustained rate. But he warned: "Sudden upturns in demand can lead to a boom-and-bust cycle, which has destabilising effects on the industry as a whole."