I'm replacing an 8-foot window with French doors leading to the existing patio slab. The house slab foundation extends 8 inches above the patio, so I need to pour a concrete step. I have already chipped away the exterior brick to expose the house slab. I know I need to attach the step to the house slab with rebar, but do I also attach it to the slab, or do I use a membrane between the step and patio to let the step float on the patio?

You should always use an expansion joint between a fixed foundation and a floating slab. The only way to tie a floating patio to a fixed foundation is with rebar THROUGH a felt expansion joint so the patio can move somewhat but still maintain its designated height location. The rebar acts a knuckle/swivel joint. That being said..


The house slab foundation extends 8 inches above the patio, so I need to pour a concrete step.

An 8" step is not code here in height in general access.

The French door has to have a poured concrete sill the width of the foundation itself. From there is the pickle, the 8" drop to patio.
The situation you are in there is less then a half step. You have the 8" above the patio and then the sill for the French door. That finish sill will more or less be the target height as the actual; "top" of the step. I have argued this with the inspectors that is measuring off the very outside tip of the concrete sill and not the threshold height which changes it as much as an 1 1/2"

Check with your inspector on the designated heights as the actual finish height of the last "step" which is the actual French door threshold. The threshold can be a player here because you could use more height. An 8"high step is to high and a mini step to lessen that is a tripper. There would only be a 2" or so high step off the patio to achieve the 7" or 7 1/2" normal riser.

Thanks... I hadn't even thought about the drop height. I did check the building code, and it says no more than 8 inches from the top of the threshold to the landing, so I'm going to have to rethink that.
But let me revisit the attachment part. I want to bond the step to the house slab because it is also the door sill, so I am using rebar drilled into the house slab plus a bonding adhesive. I want the step to float on the slab, so you are saying I should use an expansion joint plus rebar between the bottom of the step and the patio?