I presume because he designed the building after he became a teacher.

He designed it before that episode, he was given the project because of that design.
I guess that in seasons 2-4 his firm built this company for him. Maybe.

when was it stated that he never designed/built a building?
wasnt his design of the penis tower used and thats the one he shows his kids?

You might wanna watch the episode before posting an argument.

ewwwwww unfriendly people on the HIMYM spot..

Why unfriendly? I'm merely saying you shouldn't post if you don't know what you're talking about.

He never designed a building in New York (which was his dream) but in season 2, he designed the Spokane National Bank Building (the building Druthers designed that looked like a penis). This was the building Ted pointed out to his kids on their trip to Spokane.