I would calculate the load on the new lintol and install cantilever props on the inside, ie four acrow props alltogether, the two furthest will take some uplift but if they are a few metres apart the loads will not be excessive.
The neighbour must submit a Party wall award which deals with boundary issues. Write to them and ask them to comply with the Act, when they do you could dissent and have your own party wallsurveyor deal with the boundary issues.
Many alterations eg loft conversion will require a structural engineer to design the steel beams and new floor. Existing lofts are usually designed for 0.25kN/m2 and new floor should be designed for 1.5kN/m2. I would ask a structural engineer to inspect.
ga consult
A one brick thick wall (225mm) on a 450mm deep 400 x 250 thick concrete froundation should suffice for retaining 0.5m. In corporate weepholes at 1m centres and provide movement joints at say 3m to 6m centres to allow for movement.
