Wolves have reportedly had a bid in the region of £7million accepted by Blues for the 28-year-old centre-back.
Johnson said: "We're half-way there.
"It wasn't an easy decision. I enjoyed my time at Birmingham but relegation half made the decision for me.
"I want to be in the Premier League and I'm sure everyone can appreciate that.
"But I enjoyed my time at Birmingham."
Johnson has made 88 appearances for Blues since moving to St Andrews from Cardiif City in the summer of 2009.