WITH
--  Select Features
    feat AS (SELECT DISTINCT fid AS building_id, wkb_geometry AS geom FROM inside_raster
        WHERE fid = 'osgb1000005284805'
    ),
--  Clip band 1 of raster image to the boundaries of buildings
--  then get statistics for these clipped regions
    b_stats AS
    (SELECT  building_id, (stats).*
        FROM (SELECT building_id, ST_SummaryStats(ST_Clip(a.rast,1,geom,-9999,true)) AS stats
            FROM height_raster a
                INNER JOIN feat
            ON ST_Intersects(feat.geom,rast)
        ) AS foo
    )
--  Summarise statistics
    SELECT building_id, count As pixel_count,
        min AS min_pixel_val,
        max AS max_pixel_val,
        mean AS avg_pixel_val,
        stddev AS pixel_stddev
    FROM b_stats
    WHERE count > 0;

WITH values as (
        SELECT (ST_DumpAsPolygons(rast)).val AS val
          FROM rast
     )
SELECT Count(*)
  FROM values
 WHERE val < threshold