Plagarism

What do you mean by "house style"? Is that the example fragment of code that they gave to you? It should really only be used as a guide or foundation, you then base the rest of your website around it, building on it and using the materials you learned from the course.

TurnItIn uses a database of all submitted courseworks and online sources and matches to see if your work is similar to the ones in the database. Code is probably less flagged up because coding styles can be similar but that doesn't mean you can just go on the editor in Google Chrome and just rip all the code from a website. If it visually looks similar then it depends how you got there, if you used the exact same CSS styles with the same margins, padding, etc... then you're running dangerously. You can use sites as a guide but nothing more, code the rest yourself using your own creativity.