I need to raise the extension kitchen floor level to the rest of the property.

Hi Aaron, I have seen those kind of problems in a property and I can tell you what we have done, but your case might be different. The floor was poured directly on gravel, with no no insulation under it. The client experienced mould and moisture problems with the floor. We suspected no membrane under, but we discovered there was a damp proof membrane under the slab. The moisture was due to condensation. You have 3 options: 1. You add insulation over the existing slab and pour an unbonded reinforced screed on top. This adds around 130mm to your floor height (50mm insulation + 80mm screed). You can go with a thinner insulation, but I wouldn't recommend anything under 30mm. If the floor is very uneven, a self-levelling compound might be needed before the insulation. Cost guesstimate (with self-levelling): 1,500. 2. You can add insulation and a new chipboard/OSB3 tongue and groove floor. This will add 70mm to your floor (50mm insulation + 20mm floor boards). Again, if the floor is very uneven, a self-levelling compound might be needed before the insulation. Cost guesstimate (with self-levelling): 1,200. 3. Remove the existing floor (I assume it is poured concrete), add insulation under it and pour to correct height. Cost guesstimate (including 1 skip): 2,000. With regards to the door, it depends on the situation, you might be able to raise it (along with the lintel over it) so that it comes to the correct height of the floor or if it has a large threshold (assuming it is made from UPVC), it might need no alteration. With this client, they decided to go with Option 3, as it ensures everything is done correctly and avoids problems with further uneven settlement of the slab. I would advise a small trial hole through the slab to see whether it has insulation, if it does, I a screed/self-levelling to bring it up to the rest of the house, but the moisture problems make me think this will not be the solution and will persist. Best of luck, Cosmin

Without actually seeing this job no one can give you a proper answer, if they do itll be a guess at best. So ask a builder to come and look and give you a price and you'll have all your answers.