Extending loft hatch

Was thinking of putting a bigger loft hatch in for easier access. Wanted to extend the side opposite to where the ladder is leaning. I'm not 100% on what I need to do so would appreciate some guidance.

So the red lines are the existing joists which are 34mm x 98mm roughly. Green lines are proposed new hatch. Blue lines are proposed new 'braces' to compensate chopping the bottom red joist. They'd be screwed in to the joist the ladder is leaning on, and the top red joist. And then into the sides of the cut joist.

I would then put the new wooden 'frame' in where the green lines are securing it to the new braces and joist the ladder is leaning on

Is this ok? Do I need another 'brace' above the top horizontal green line? Or have I got it all horribly wrong?

 

You need to avoid cutting out any part of the prefabricated roof trusses.

 

Does that extend to the joists? I'm not planning on cutting anything other than the bottom joist in red!

 

The 'joist' you're planning to cut is the bottom chord of the truss.

The truss is designed (engineered) to act as a complete unit - removing or cutting one component transfers load to the other components - the bottom chord is, in simple terms what holds the top chords in position. Cutting the bottom cord could allow the top chords to spread.

 

which pushes the walls of a house apart.

If you were to enlarge the hatch "east and west" rather than "north and south" in your first pic, you could have a wider hatch, easier to climb through, and only have to cut plasterboard, not timber.