Failing query, don't understand why
ST_Distance_Sphere has first argument (SELECT coordinates FROM houses WHERE id = 7) then you close ), so (SELECT wkb_geometry... is second argument for round, which I suppose is not what you want.

So I believe you just got lost in parentheses - try rewriting code "in ladder" so every parenthesis has closing one in same indentation... 

based on your working query - are you trying to do this?

SELECT round(
  ST_Distance_Sphere(
    ((SELECT coordinates FROM houses WHERE id = 7 LIMIT 1)
  ,
  (SELECT wkb_geometry FROM coastline ORDER BY wkb_geometry &lt;-&gt; (SELECT coordinates FROM houses WHERE id = 7) LIMIT 1))
)) AS DISTANCE;