Welcome and thanks for joining the Community, @themill.
I'm here to share some information about the report you'd like to pull up in QuickBooks Online (QBO).
You can run or pull up a Profit and Loss report. This will show all your income and expense transactions. However, QBO currently doesn't have the option to show them as "cost per gross square foot" or "cost per leasable square foot." We are also unable to add a square footage column. You can export the report to Excel and customize it there as a workaround.
1. Go to Reports menu at the left pane, then enter Profit and Loss on the search bar.
2. Choose your preferred date on the Report period section, then select the Run report to refresh.
3. Click the Export icon (a box with a right facing arrow) between Print and Gear icons, then Export to Excel.
Just in case, you can also check out this article for future reference: How to customize your Profit and Loss report.
This should answer your concern for today. Please let me know how it goes by leaving a comment on this thread. I'd be glad to answer if you have any follow-up questions. Take care and enjoy the rest of the day!