Chadwick Lawrence is re-locating from Bull Close Lane, Halifax, to the Rawson Street Georgian building by the end of the year once the £250,000 renovation of the first floor is complete.
Developer Chris Turczak, of Tyler Projects (Halifax) Ltd, was given the £2 million task of renovating Grade II listed Somerset House five years ago.
As part of the project the Grand Salon was restored as a ceremony room for weddings, although last month Calderdale Council announced registrars' functions would now be centralised at Spring Hall Mansion, Halifax.
Now the so far undeveloped first floor is being prepared for the new tenant. "This floor has been waiting for the right type of occupier," said Mr Turczak.
"We always thought an office would be ideal here. We couldn't start on the work until we knew the specifics required for whoever moved in as it's a complicated building."
The economic climate has also been a reason for the delay in completion.
"We're now in the final stages of the Somerset House development," said Mr Turczak.
Work started on the floor about five months ago, to transform what is two back-to-back buildings into one for the firm. This part is believed to have originally been where wool was kept, with below as access for horses.
Nick Worsnop, partner at Chadwick Lawrence, said the firm, which also has offices in Leeds, Wakefield, Huddersfield and Dewsbury, had been looking for the right site for around five years.
Moving in will be the 15-strong Halifax team, covering commercial ,matrimonial, personal injury and conveyancing.
Mr Worsnop said: "This is a more central location for us and will help us drive the business forward."
As soon as this project is complete, Mr Turczak will move onto finishing the apartments upstairs - work which was shelved due to the recession.