I had the same problem when I was looking for a trenchcoat pattern a few years back. My solution... I just took a suit jacket pattern and extended it. Worked perfectly. Just add the same amount all across the bottom of the pattern pieces. Just remember that if you are planning to wear this over heavy clothes (such as a suit) you need to go a size up from your size. For instance, if you wear a size 40R, you need to cut your coat in 42R.
Of course if you intend to have a form fitted trench, one that you just wear over a shirt and pants for instance, you can ignore this suggestion.