Is it considered plagiarism if an author reuses their own ideas from previous works in their new novels?

No, certainly not. I was asked to write for a TV show called The Great Detective produced by the Canadian Broadcasting Corporation. After being commissioned and writing one script, I put together spec submissions for five other “Great Detective scripts. The ideas were welcomed by Producer Peter Wildblood and the story editor, John Maynard, said he would have some good news in a few days.

Sadly the news was not good as the series was cancelled. I mothballed those script ideas and eighteen years later pulled them out and created a new leading man, Detective Quentin Nickles, and used the bones of those five scripts to create a new series for CBC Radio Drama which ran under The Mystery Project at CBC. The Investigations of Quentin Nickles went on to live for 20 episodes which ran on CBC Radio all across Canada several times. The series was then bought by the BBC in London from what I understand.

So, though my spec stories were not used for CBC TV back in the 1980s, they eventually saw life as CBC Radio drama episodes.

Now I am considering taking those twenty radio stories I created for radio drama and turning them into novels to reach a new level of audience.

CBC has also released 17 of the 20 shows as Sanwal CBC Radio where they can be freely downloaded and listened to on your computer or in your automobile as you drive.