  In this two-part paper we prove an existence result for affine buildings
arising from exceptional algebraic reductive groups. Combined with earlier
results on classical groups, this gives a complete and positive answer to the
conjecture concerning the existence of affine buildings arising from such
groups defined over a (skew) field with a complete valuation, as proposed by
Jacques Tits.
  This first part lays the foundations for our approach and deals with the
`large minimal angle' case.
