The phrase "are already extensive" is correct and usable in written English.
It can be used to describe something that has already reached a significant level or amount, often in the context of information, resources, or experience.
Example: "The research findings are already extensive, providing a solid foundation for further studies."
Alternatives: "are already considerable" or "are already substantial."
It may well be, but there are already extensive powers within the succession of criminal bills passed by David Blunkett.
They have not tried large-scale exports of fully assembled vehicles to industrialized countries, although their exports of auto parts to the West are already extensive and growing.
We lift up the 2015 Parliament of the World's Religions Interfaith Call to Action on Climate Change: "The damaging impacts of climate change are already extensive...If human behavior does not change, these impacts will become far more extreme, resulting in turmoil and suffering on an enormous scale with immense harm to both humans and other forms of life.
Often, these lesions are already extensive in early and intermediate stages of the disease.
This is likely to be more evident in elite athletes, who are striving to be the best of the best, where training levels are already extensive.
Transillumination to identify caries lesions in anterior teeth has been used for a long time; however, such detection is limited to identifying lesions that are already extensive in their progress through the enamel on the way to cavitation.
Werritty's connections in Sri Lanka were already extensive.
The training for officers in the unit is already extensive, with duties ranging from hostage negotiation to biohazard detection.
Doctors rely on symptoms to make the diagnosis, and most think that by the time symptoms show up the brain damage is already extensive.
Stephen Yane-Loehr, an immigration law professor at Cornell University, is skeptical of Trump's promise to enhance a process that is already extensive and lengthy.
Prior to the recent crisis, U.S. sanctions on Myanmar were already extensive.
