Hey @maxgibbons, I’m Kaus from Unbounce Support 

At this time, Smart Builder and Classic Builder are not interchangeable as they’ve got quite different interfaces as I’m sure you have seen! Can I ask what kind of code you’re looking to add? You could technically add a custom script to your page in Smart Builder, but it will have to be done through Script Manager at the domain level. You can read a bit more on this here: https://documentation.unbounce.com/hc/en-us/articles/4404641228052-How-Can-I-Add-Custom-Scripts-onto-my-Smart-Builder-Page-

If you’d prefer to add the script to the page individually and want to proceed with Classic Builder, you’ll need to create a new page. You can click “Create a New Page” from the top right of your all pages overview. You’ll be prompted to select either Smart or Classic builder, and here’s where you’ll want to select “Classic” to proceed :)