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Schengen and 183 days, together
For second-home owners in Spain, France or Portugal with a UK or other non-EU passport: the same stays, counted against the Schengen 90/180 rule and against the 183-day tax test.
Why the two rules meet
With a UK or other non-EU passport and no residence permit, the Schengen rule allows at most 90 days in any 180. In practice that means two stays of up to 90 days a year, about six months apart. So the 90/180 rule usually stops you before you get near 183 days in the country, and the 183-day tax test only comes into play if you hold a residence permit or a long-stay visa.
Once you hold a residence permit there, days in that country no longer count towards your Schengen 90, but they still count towards its 183-day test. Spain counts more than 183 days in the calendar year; Portugal more than 183 days in any 12 months; France looks at where your main home and life are, with 183 days as the usual guide. See the 183-day rule by country.