The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
11. Where concentration of mind (is possible), there (meditation may be carried on), on account of there being no difference.
A doubt here arises with regard to direction, place, and
time, viz. whether any restrictive rules exist or not.--Against the view of those who maintain that such rules exist because we have analogous rules concerning the locality, &c., of Vedic works, the Sûtrakâra remarks that all rules concerning direction, place, and time depend on the aim merely; that is to say: Let a man meditate at whatever time, in whatever place and facing whatever region, he may with case manage to concentrate his mind. For while scripture prescribes an easterly direction, the time of forenoon, and a spot sloping towards the cast for certain sacrifices, no such specific rules are recorded for meditation, since the requisite concentration may be managed indifferently anywhere.--But, an objection is raised, some passages record such specific rules, as e.g. the following one, 'Let a man apply himself (to meditation) in a level and clean place, free from pebbles, fire and dust, noises, standing water, and the like, favourable to the mind, not infested by what hurts the eyes, full of caves and shelters' (Svet. Up. II, 10).--Such particular rules are met with indeed; but the teacher being friendly-minded says that there is no binding rule as to the--particulars mentioned therein. The clause 'favourable to the mind' moreover shows that meditation may be carried on wherever concentration of the mind may be attained.