Extension floor is higher than existing house

Ask for a level survey — a builder, surveyor, or screeding specialist can use a laser level to document how much the floor is actually out. Refer to the tolerances in BS 8204 — this gives you clear grounds to dispute the “acceptable” claim. Discuss remediation — either removal and re-screed, or partial correction if you can live with it.

A 50mm rise across 3m is quite noticeable and not ideal for an internal floor. Normally, a maximum deviation of around 10–15mm would be acceptable for domestic settings. Grinding down concrete can help slightly, but 50mm is too much to remove safely that way — it may weaken the screed. The best approach is to assess whether the screed depth and damp proof membrane allow partial re-levelling or replacement to achieve a flat transition. I’d recommend a site visit to check the floor build-up and provide a levelling solution.”

It is not generally acceptable for a new extension floor to finish 50 mm higher than the adjoining existing floor unless this was specifically detailed in the design or agreed with you prior to construction. Current good practice in extension work is to achieve a level and continuous finish between new and existing areas, ensuring safe access and avoiding perceptible slopes. A 50 mm rise across 3 m equates to a gradient of approximately 1:60, which is significant enough to be noticeable underfoot. Grinding down the concrete or screed is rarely advisable, as this can compromise the integrity of the floor, the reinforcement (if present), and any damp proof membranes. It may also create additional unevenness. The appropriate remedial action depends on how the issue arose: Check approved drawings and building regulations – confirm the intended finished floor levels. Discuss with your builder in writing if the agreed specification was for level floors, this should be rectified at their cost. Possible solutions include re-screeding the floor to correct the levels, installing an appropriate levelling compound to transition smoothly, or in more severe cases, partial removal and reinstatement of the screed. You should not accept a significant step or slope unless it was unavoidable due to drainage or structural constraints and was clearly communicated beforehand. I would recommend requesting a joint inspection with your builder and, if necessary, involving Building Control to ensure the floor complies with regulations and accepted tolerances.

I think Lionvale Constuction's advice is on point here. I can't see how or why a finished floor should be anything other than level, flat, and at the same height as the existing floor level. Removing the offending floor screed and replacing would be the desired approach. Potential issues arise with the subfloor and insulation given the excessive difference of 50mm. Screed is usually about 65mm thick on top of any insulation, and it sounds like the block and beam slab is out, or if it's a ground bearing slab the same applies, but GBS would be easier the rectify. There is potentially quite a number of knock-on effects to rectifying this issue.

It sounds that your builder has made an error in his levels wouldn't pay him until he rectifies this 50mm out over such a small erea he's got to read a level

No this is not expected! All new floor levels should match the existing always within a mm tolerance, you should need to do anything with your existing floor unless it actually is out of level and want to correct.