I found another passage that may shed light on the pāsāda. In MN 12 we have:
Seyyathāpi, sāriputta, pāsādo, tatrāssa kūṭāgāraṃ ullittāvalittaṃ nivātaṃ phusitaggaḷaṃ pihitavātapānaṃ.
Suppose there was a stilt longhouse with an upper storey plastered inside and out, draft-free, with bolts fastened and windows shuttered.
If we take this at face value, then the actual enclosed upper storey of a pāsāda was referred to as kūṭāgāra. Of course, this makes perfect sense, but it does raise some questions. Are all kūṭāgāras in fact the upper storeys of pāsādas? It seems unlikely, but we should look out for it. And how best are we to translate it in such cases? Ven Bodhi has “upper chamber” which seems inadequate.
But it’s not entirely clear to me this reading should be accepted. The description of the luxurious kūṭāgāra is stock (eg. AN 8.30). Has it simply been dragged along to fill out the description of a luxurious dwelling, which here is applied as a metaphor for heavenly pleasure?