In the context of the OTU, from the 'canon' sources, if you will, not too much terraforming has been done by human empires.
First, given the nature of the setting, there is plenty of real estate, so people live where it is easier to live. Since many ssytems have a 'livable' mainworld, terraforming is less attractive.
Second, the Vilani did not have the tech to do it as they only made it to TL11 (Mongoose list terraforming as TL12). It is only with the coming of the Terrans and achieving TL12 during the Interstellar Wars, that it becomes possible. Unless the Ancients were doing it. But then they were the last owners of the real estate that is Charted Space.
So terraforming can be done, just not within a timeframe that most governments can survive long enough to see completion.
First, given the nature of the setting, there is plenty of real estate, so people live where it is easier to live. Since many ssytems have a 'livable' mainworld, terraforming is less attractive.
Second, the Vilani did not have the tech to do it as they only made it to TL11 (Mongoose list terraforming as TL12). It is only with the coming of the Terrans and achieving TL12 during the Interstellar Wars, that it becomes possible. Unless the Ancients were doing it. But then they were the last owners of the real estate that is Charted Space.
Finally the the time and technology implied on the section on Mars and the terraforming there, it will take thousands of years to thicken the atmosphere.Hephaistos is [bold]one of the few completed terraforming projects in the Imperium.[/bold] Begun during the Interstellar Wars, the project was abandoned and resumed several times. The project was completed by the Hephaistos Company, chartered by the Imperium in 632. The planet was opened in 835, and sections were sold to several colonising groups. Although the project is officially complete, the company is still engaged in work to reduce the ocean and atmosphere. - Solomani Rim p.106
So terraforming can be done, just not within a timeframe that most governments can survive long enough to see completion.