By the way, if you need to make just your first survey, you can download Ekahay Site Survey evaluation, and it will work for 15 days same way as licensed version. You just need laptop with supported Wi-Fi adapter, and there is just one (but important) limitation.
You even can create and save report for survey with trial version. What you can't do without license -- is to save survey data. So you want to make report after surveying every part of the building, and save it. It isn't as good as having full survey data, but it is quit a lot to get for free.
And, yes, you need building plan for survey. It is enough to use photo of evacuation plan, which is presented in every public building, and you need to calibrate it with length and width. Building size you can measure using Google Earth, or just measure before survey. In the worst case very rough plan made in Paint will do as far as size is correct and you made survey properly.