The asking price for the building on Avebury Boulevard is £5,850,000.
Much of the work has already been done, with many of the flats finished. But there is still consent to build two extra storeys, which would provide a further 14 flats.
A spokesman for Robert Irving Burns said: “The conversion of 32 flats has been completed, with 19 units having already been sold on long leasehold interests, and the remaining 13 flats having been built but are unsold.
"The additional 14 flats have not been completed, with further work required,” he added.
It is not clear whether all the flats will eventually be sold or whether any will be for rent or affordable rent.
Currently developers can carry out some office to residential conversions under the government's 'permitted rights' legislation and they do not need planning permission from the council.
This mean councils have little say in what is happening, no Section 106 funding for the community and also no power to insist the developer provides any affordable or social housing in their conversion scheme.
The action had been partly prompted by the scheme to convert the offices above CMK rail station into 200 flats. This was refused permission by the council after cross-party opposition by councillors. But, following an appeal, the developers were last year granted permitted development rights to go ahead.
Council leader Pete Marland had previously described the proposed Station Square flats as "poky", saying they will be sited above a busy and noisy railway station" and have “windows that do not even open”.
The new order will ensure better quality developments in the citycentre, he said.
“The government’s inconsistent and confusing planning rules can lead to bad development, but this decision means we’re able to protect Central Milton Keynes’ buildings and economy, and fight for good quality homes for our residents.”