The classical Menger-Urysohn addition theorem states that, given two subsets $A$ and $B$ of a metrizable space, we have $dim (A cup B) leq dim A +dim B +1$. Jerzy Dydak's extension theory analogue was the following: given two subsets $A$ and $B$ of a metrizable space, and two CW-complexes $K$ and $L$ such that $A au K$ and $B au L$, we have $(A cup B) au K ast L$. We generalize this further using stratifiable spaces instead of metrizable ones. The class of stratifiable spaces lies between paracompact and metric spaces.

Keywords: covering dimension, absolute extensors, CW-complexes, stratifiable spaces