First post, since honestly I usually just browse here, but since we ourselves are currently building a kit house with Ytong and are already quite advanced, I wanted to respond to you once. For context: I am building in northern Germany and do not know the prices in BW exactly, but since we have a kind of north-south gradient, I would feel like saying it should be more expensive for you than for me. Our house is somewhat larger in area but comparable and, like yours, includes an extension, somewhat larger (double garage 6x9m).
Our kit cost about €150,000 with the standard scope of services of the Ytong kit house. We do or have done a lot of work ourselves and have certainly saved a lot, but as 11ant wrote, the amount is significantly lower than you are assured and the time you spend on the construction site and alongside it should definitely not be underestimated. I don’t want to discourage you, I would do it again as it is/was a challenge and especially fun when you see what you have accomplished.
There are a few things in your offer that I can’t quite understand and would suggest you check again, because there are enormous additional costs hidden there that significantly reduce your savings. The Ytong kit house includes the closed shell as a basic service. The intermediate ceiling is delivered and set up with you (you have to provide 3 helpers), the roof truss is delivered and erected by a specialist company, and doors and windows are delivered and installed by a specialist company (except skylights). Everything else is just material deliveries. However, this basically does not include (so this should be checked by you) materials you otherwise need: mortar for the first stone layers, concrete for the ring beams (ceiling and walls), lintels, membranes, roof and floor insulation, etc. These are all extra costs found under one of the extra points in the total cost calculation. I also can’t imagine the foundation slab being included in the kit price, it is only one cost item in the total costs, as is the roofing of the roof truss. In the kit, you actually only get the gutters, roof tiles, and skylights delivered. We had a roofer install these including cladding of a bay window and excluding the interior finishing, and here we are already at €40,000. Additional costs you still need to consider include, for example, securing with scaffolding, which usually stays up much longer than planned, because on the one hand you quickly overestimate yourself in terms of work progress and on the other hand you have no influence on many other factors. If your ceiling comes not in 4 weeks but in 8, your roof package not in 2 but only after 8 weeks, the carpenter can only give you a free appointment later, etc. This quickly adds up to a few weeks and a few thousand euros more. When the ceiling is set up, a crane is used, which is included up to a certain size. If a larger one is needed, it is at your expense.
Much of this was initially unknown to us and Ytong did not point it out either, so you stumble over it quickly. Moreover, in the cost calculations I have seen so far from other self-builders, often far too few extra costs for the individual trades are included, because they partly have no offer for the work from a company or they calculate with an absolute minimal equipment (e.g., a bathroom completely from the hardware store) and of course the numbers then look smaller and nicer. In other things that you have included directly in the kit, it is just like in turnkey building, you have many surcharges after an 'inventory check' (ceiling outlets, windows, doors, etc.), you have to keep that in mind.
Please also erase the illusion that you will get a lot of help. The services are either clearly defined as not doing something or vague in that they do not do it. For example, you get a foreman who sets the first row of stones on the ground floor, only the exterior walls. If you then have questions or need help, they charge a daily rate of about €600. On the upper floor, I believe nowadays only the corner stones of the first row are set (this is stated in the contract). Construction supervision also does not mean that once a week someone comes who looks at your progress and tells you how to proceed or what you have to consider. Usually planners sit there and not executors. For example, many things could not be told to us on request about how or with what they have to be executed, only how it looks afterwards in the drawing and the shell construction is not only about laying Ytong stones. This usually does not cost you money to find out, but it takes a lot of time as well as the organization of all other building materials and the coordination of other trades, since this is not done by Ytong BH, but by you.
So that was a lot, maybe even a bit of a rambling text :D
In summary for your question: Look at the offer again carefully, what is really included and what is not explicitly included. Talk to your Ytong BH representative and clearly define things; the more they break down everything for you in advance, the fewer surprises you will have later. But be aware, you will not be able to think of everything and usually you no longer have contact with the representative afterwards, but only with a planner who was not involved in the original discussions. And should it come to a contract, have maximum delivery times and response times committed to you and make sure every extra you wanted from the outset is also listed. The planning drawing on which the offer is based is usually not part of the contract.
If you consider all that and inquire locally about the costs of certain things in approximately the quantities you need (here my raw material dealer helped me a lot who is very committed there) and add that to the further costs for the urgently needed trades (roofer etc.), then compare that with 1-5 other offers from building partners (turnkey) which you should get. Comparing shell construction itself is only possible if you ask for the trades individually from a construction company.
You will certainly save, but pay with a lot of your time. I did it because we wanted to do some things ourselves like electricity and already had known building partners for many things (sanitary and roof truss).
I hope I could help you a bit. If you have questions, feel free to contact me.
Kind regards