Query a Custom Post Type to another CPT via Metabox

Question

You’ll want to create a standard metabox via the official APIs, how to do that is beyond this answer, what I’ll talk about is your dropdown.

What you need to save is the Post ID of the building, so something like this pseudocode:

<?php
$current_building=....;
?>
<select name="....">
<?php
$query = new WP_Query( array( 'post_type' => 'buildings' ));
if ( $query->have_posts() ) {
    while( $query->have_posts() ) {
        $query->the_post();
        ?>
        <option value="<?php the_ID(); ?>" <?php selected( get_the_ID(), $current_building ); ?>><?php the_title(); ?></option>
        <?php
    }
}
?>
</select>

Now that you have the input sorted, treat it as any other input in a metabox. The saving and creation of the box is the same as normal