Building Brokers generally (check that - it changes case by case) own the copyright over your plans much the same as a builder does.
the Broker then ships the deal out to a panel of builders and gets paid a commission - anything from 1.5% to 5%.
i give my clients full copyright over the plans for their block, but i retain copyright over the individual plans entirely, so if they're used again, it's pay day.
there's a big difference there.