June 20, 2008, Blitz server: At level 14, improves building rate by 24% for all buildings except barriers.
June 14, 2009, Guild server: At level 15, improves building rate by 27% for all buildings except barriers.
Therefore, building rate is increased by 3*(Level - 6)%.
This can be a very valuable ability. For instance, 299 workshops build 10 towns. With a level 15 engineer, you can have now 239 workshops that produce the same effect (8 * 1.27 = 10.16). This means those 60 acres can be used for other structures.
The engineer becomes even more valuable if you want faster build rates. With 299 workshops, a level 16 engineer is the equivalent of an additional 90 workshops since you increase your build rate by 30%.