I have a new extension and my main wall taken out my builder is 30 mm out is my extension

If it was me doing it was would take the existing floor up and start again and make the screed to the right level so it was all one level

If this is the ground floor the builder will have either used a block and beam system or a traditional concrete overiste. Either way this should have finished around 150mm below your existing finished floor level to allow for insulation and floor screed to finish nice and level with your existing floors ready for floor finishes. In some cases where adjoining to timber floors or very old concrete floors that may have damage or sunken over time it is wise to latex over the whole area before laying the finished floor. I hope this helps.

There are products on the market that can easily fix this situation. Known as levelling compound, some types can deal with thicknesses up to 50mm. It should be applied strictly to the manufacturers recommended quanities for water and compound ratio. The surface should be completely clean prior to application and most can set in a few hours although overnight is best. Have the right tools and buckets for the job and enjoy the process.

You can get self leveling with fibers in and goes upto 50mm it would cost alot to do but cleaner option but do it 3 or 4 layers to build it up