Non-residents: UK income: Permanent establishment
The general definition of a permanent establishment contains three requirements, namely
a) there must be a place of business, normally premises, although it can, in certain circumstances, be machinery or equipment,
b) the place of business must be fixed, that is, have a certain degree of permanence, and
c) the business must be carried on through this fixed place of business, normally by the personnel of the enterprise.
The OECD model agreement commentary on Article 5 contains detailed notes on the definition of a permanent establishment.