How to calculate property square footage?

[Matt Mainini]

I am building an ADU on my property and need to know the total square footage of the property. It seems straight forward, but it's a complicated measurement. Length x Width = Area right? It's not that easy unfortunately.

I have had an expensive topographic survey done, however my Architect and Surveyor struggle to provide the total square footage. The shape of the land is an irregular quadrilateral with no known angles. All 4 lengths are known. Can you please help?