We introduce notions of lax semiadditive and lax additive (\infty,2) -categories, categorifying the classical notions of semiadditive and additive 1 -categories. To establish a well-behaved axiomatic framework, we develop a calculus of lax matrices and use it to prove that in locally cocomplete (\infty,2) -categories lax limits and lax colimits agree and are absolute. In the lax additive setting, we categorify fundamental constructions from homological algebra such as mapping complexes and mapping cones and establish their basic properties.