Jared,
You may be making this more complicated than necessary, based upon your response to Jake with your list of actions thus far. All of the comments so far are useful, though offer up a few different approaches. The Spatial Join tool is your best bet for getting the PIN info into your building footprints.
You don't say whether your existing building footprint layer must stay or if you can simply remove and replace it with an updated version. Jake's comment regarding joining the Spatial Join result back to the original layer is operating on the assumption that the existing layer must stay. I will second Chris' warning about using the OBJECTID as a unique ID, as you have no control over the system reordering the IDs, so the suggestion for creating your own unique ID would be useful IF the existing layer must stay.
But, let's keep things simple and assume that you can simply replace the existing buildings layer with the end result new buildings layer once you've finished processing.
- Set up your attribute fields in your buildings layer however you want or need them. This presumably includes a Parcel PIN field. I suspect this is already done.
- Fire up the Spatial Join tool from ArcToolbox. From your original post, you want to attach the PIN TO the building footprint. That's the "clue" for deciding, in Spatial Join, which is the Target layer and which is the Join layer. Set your output feature class (this will be your new buildings layer). Keep all Target features (because you set up your buildings layer in step 1 the way you want it. In the Field Map of Join Features box, delete any fields from your parcel layer (Join features layer) that you don't need (don't delete any of the buildings layer fields!). If you are only transferring the PIN, delete all of the parcel layer fields except the PIN field. This will save you from having to delete the extra fields later. The Intersect match option usually works well. I'm assuming that your building footprints don't straddle multiple parcels and are fully within a single parcel. Click OK and run the tool. Your new feature class will be created.
- In ArcMap, open the attribute table for the new feature class, right click the PIN field you created in your buildings layer (the empty field) and select Field Calculator. Find your parcel layer PIN field and double-click it to put it into the query box. The query effectively is 'BuildingLayerPINfield = ParcelLayerPINfield'. Click OK and whatever value is in the ParcelLayerPINfield will be transferred over to the BuildingLayerPINfield. If you had other info from the parcel layer that you wanted to transfer, perform a similar operation on those fields.
- Delete the ParcelLayerPINfield from your layer, as you don't need it any longer now that the info has been transferred. Delete any other parcel layer fields you may have kept. The end result should be a buildings layer that contains only the attributes you originally created.
- This buildings layer is now your new buildings layer. You can get rid of the old one.
That takes care of the PIN field. It sounds like you can do some group selects to populate other fields. Adrian mentioned the Transfer Attributes tool. I use it in certain workflows, but keep in mind that the tool is used in a one-to-one fashion. After setting up the fields you want to transfer, you select a source feature and then a target feature and the mapped fields' attributes are transferred. You could use this for the parcel PIN but you stated you didn't want to do them one at a time. That makes it the "wrong" tool for your current purpose. I only have a cursory knowledge of the Attribute Assistant tool, so I can't comment regarding its usefulness in this context.
Brian