106,930 new buildings would be inserted with addresses (green) (non-conflated, with addresses)
139,987 new buildings would be inserted without addresses (non-conflated, no address)
18,867 buildings already exist with addresses and would not be inserted
13,226 buildings already exist without addresses and would not be inserted

Here are sample screenshots of Sonoma County, Santa Rosa, the City of Sonoma, and Petaluma:

Obtaining Data

original_data from:

Overpass query (you may save as OSM file, shapefile, or postgres sql dump depending on your overpass client)

area[name="Sonoma County"];
(
  way[building](area);
  relation[building](area);
);
(._;>;);
out;

If using an Overpass -> QGIS -> Postgres dump, save it as osmquery_buildings_pgdump.sql for later. Otherwise osm2pgsql should create tables like son_polygon for later.

Prerequisites

The postgis package appropriate for the version of postgres server you have installed (in my case, 11)

Ubuntu

sudo apt install postgresql-11 postgresql-11-postgis-3 shp2pgsql osm2pgsql

Debian (shp2pgsql is included in postgis)

sudo apt install postgresql postgis osm2pgsql
The postgresql server started/running/configured and database gis created

Running

Run the following SQL as a superuser (postgres) inside the gis database to enable the PostGIS and hstore extensions: CREATE EXTENSION postgis; CREATE EXTENSION hstore;
Unzip the original_data and open a shell in that folder.
Here we are assuming that county data is in WGS84/EPSG4236 format, which was true as of last check and is also what OSM uses.
Run from your shell: shp2pgsql -s 4326 -I Parcels__Public_.shp | psql -d gis -U postgres -W
shp2pgsql -s 4326 -I Sonoma_County_Building_Outlines.shp | psql -d gis -U postgres -W
shp2pgsql -s 4326 -I osm-buildings-01-03.shp | psql -d gis -U postgres -W

Now all the data is in Postgres. For processing and conflation, read through and execute conflation.sql as per your comfort level.

Internal Notes

http://download.geofabrik.de/north-america/us/california/norcal-latest.osm.pbf

shp2pgsql -s 4326 -I Parcels__Public_.shp | psql -d openstreetmap -U openstreetmap -W -h localhost -p 54321
shp2pgsql -s 4326 -I Sonoma_County_Building_Outlines.shp | psql -d openstreetmap -U openstreetmap -W -h localhost -p 54321
psql -d openstreetmap -U openstreetmap -W -h localhost -p 54321 -f osmquery-pgdump.sql

#unused
osm2pgsql -d gis -c --prefix son --slim --extra-attributes --hstore --latlong sonoma-orig-buildings-20201219.osm -U postgres -W`
osm2pgsql -d openstreetmap -c --prefix son --slim --extra-attributes --hstore --latlong norcal-latest-20200103.osm.pbf -U openstreetmap -W -H localhost -P 54321