Building Renovation - Nominal Code

Hi, Ive just started bookkeeping for a commercial property company, mainly leasing, however the company also owns the entire building from which we operate out of, we lease the bottom floor to various businesses and the rest of the building above is being renovated into serviced apartments.

During a meeting the MD discovered that the last bookkeeper had not set up a separate nominal code to record all of the costs associated in rennovating and repurposing the building, they were all going into a general expenses account, a 5XXX nom code as we recharge to the tenants, for the entire building, including the leased bottom floor. She was then keeping a record in an excel spreadsheet of the costs just for the renovation. I have the task of setting up a new nominal code to transfer all the costs from the general expenses and excel into Sage 50. My question is, what nominal code category should I use. Would it come under fixed assets?

Thanks in advance for any help.