Allen & Overy (A&O) has secured a £75m bank facility to help cover the cost of its new offices at Bishops Square.
The loan was put in place last year to augment the current £65.8m cash surplus A&O has built up via retained profit to pay for the move, planned for this October.
A&O finance director Ian Dinwiddie said the firm currently had no bank debt, but was now "getting into the period of heavy spend", with 800 contractors on-site.
"We already have a significant fund to pay for the fit-out, but we expect we'll draw down on the facility to an extent, depending on trading conditions," said Dinwiddie.
It is thought the amount borrowed will be around £40m.
A&O's accounts show that the firm has also created a minimum capital layer for the first time, following the introduction of a new accounting adjustment (FRS25).
Partners' capital currently classified as equity, the minimum capital layer, is £35m. The magic circle firm has reclassified the remainder of its partners' capital (£74.5m) as a liability, putting total partners' capital at A&O at £109.5m.
Dinwiddie said A&O had modified the partnership deed to create the minimum capital level. If partnership capital were to fall below £35m as a result of repaying partners their money, there would now be no automatic right to repayment. "The decision to repay would be down to the board," he said.