Builder’s warranty and alterations

We want to change a doorway to a room in our recently bought house (constructed less than a year ago by the first owner). The builder is not interested in helping us as they don’t deal with post contract/handover alterations, so we are on our own.

We want to move a standard doorway (820mm wide) to a ground floor room (of a two story house) about 1.5m to the center of the wall and make it a double French door (~1500m wide).

When comparing the original drawings against those available on builder’s website for our particular home design, I realised that this bedroom was created by closing off an open living space and adding built in robes. So I’m guessing the wall is not loadbearing because the plan on the website doesn’t have a wall there at all. But I will seek professional advice on this before I get someone to make a big hole in the wall.

My first question is how will such a modification affect the builder’s warranty?

So far I haven’t spoken to any contractors, so no idea how much it would cost. Assuming the wall is non loadbearing, my second question is how much would it cost?

I’m pretty comfortable with tools and I have undertaken small reno projects at our previous property, so I can probably take care of the flooring, painting, even plastering if it makes a significant difference to the budget. What I can't (rather not willing to) do is close off the current doorway and frame a new one in.

Thanks.