Foundation for extension: Is that correct, any experiences?

Hello. This is my first post here, so I am including a brief introduction. My name is Stefan, I am 31 years old and recently bought a house built in 1951, which I am currently renovating / refurbishing and remodeling. Although I am not a professional (trained merchant, not a craftsman), I am doing a lot of the work on our house renovation myself. Right now I am gathering information for the planned extension at the front of the house (the house entrance is to be enlarged / extended outward by 1.5m and the front door rotated by 90°).

I will now describe how I have planned the construction project so far and would be grateful for tips on whether it is correct or how it could be better / more sensible / even feasible. As I said, I am not an expert, so please excuse any missing technical terms and probably often amateurish expressions.

The extension itself is to have dimensions of 1.5m depth and 3.5m width, with the front door on the short side at a 90° angle to the normal house front. The house entrance (now and in the future) is about 50cm above ground level and is reached by 3 steps.

The construction is to be built from aerated concrete or Poroton with a thickness of 17.5cm. For the foundation, an 80cm deep trench is to be excavated and concreted on the outside edges to ensure frost resistance.

Here is my first specific question: Is it sufficient to make this foundation up to "ground level" for building up to that point and then to build on top of it (with appropriate moisture protection), or should the foundation be concreted significantly above ground level using formwork? Possibly even the full 50cm that will have to be built up anyway?

How should the connection to the house or basement wall be made? Simply "concrete onto" it or is a moisture barrier required between the basement wall and the foundation for the extension? It is of course clear that insulation is necessary especially at the transition to the outside (towards the soil).

The 50cm above ground level to reach the house entrance level is to be filled with construction rubble. On top of that there will be screed, underfloor heating, top screed, and tiles (just like or continuous with the adjoining / extended corridor through the extension). Any objections?

I hope I was able to explain my project clearly, and even if my planning is misguided, I kindly ask for well-meant advice.

Thanks in advance and best regards

Hi,

I hope I was able to explain my plan clearly, and even if my planning is off track, I would appreciate well-intentioned tips.

Get a professional to plan the extension for you. Detailed planning in the forum is really not advisable. And this extension with connection to the existing building must be planned.

Regards

Hmm, I actually thought that my questions could be answered quite well with some technical knowledge. It should be relatively generally applicable whether, for example, the existing foundation must be sealed off against the new one or whether such a foundation is simply poured 10cm above ground level or preferably shuttered and fully raised 50cm high, or if it doesn’t go above the ground at all but instead a moisture barrier is applied on the outside to the masonry... I don’t quite understand now what the expert on site MUST inspect or what you mean by it needing to be planned...

But thanks anyway for your contribution.

It should be relatively universal,

NOTHING is UNIVERSAL when it comes to building in existing structures (and this includes the extension).

whether, for example, the existing foundation needs to be sealed off from the new one.....

what do you mean by sealed off?

I don't quite understand what the professional on site MUST inspect or what you mean by that it has to be planned...

Well, the foundation has to hold (structural integrity), the energy saving ordinance must be complied with (thermal insulation), waterproofing must be properly planned and executed (moisture protection). Surely the extension will get a roof, how do you connect it to the existing roof? How do you avoid thermal bridges (connections between existing insulation/new insulation and so on and so forth... do you trust yourself to handle all that?

Regards

Nothing is ABSOLUTELY certain when building onto existing structures (and that includes extensions).

Now I was assuming differently based on other crafts. If a tiler wants to lay tiles, he uses tile adhesive or mortar. If my mechanic wants to change the brakes on a car, he always uses the same tools. When it comes to masonry, I assumed that a mason laying a foundation probably always follows a similar procedure.

What do you mean by sealing off?

What I already wrote in the first post: "How should the connection to the house or basement wall be made? Simply 'concrete onto it' or is a moisture barrier needed between the basement wall and the foundation for the extension?"

Well, the foundation should hold (structural stability)

That’s why I described how it should be done and what load (size, material, wall thickness) will be applied. Here I am back to my apparent misconception that such information is enough for a professional to form an opinion, at least if my approach does not work and for what reason.

The Energy Saving Ordinance must be complied with (thermal insulation)

The building envelope will still be fully insulated, there is currently NO insulation and it will be completed after the extension is finished. Therefore, I have no questions regarding the required thermal insulation.

Sealing must be properly planned and executed (moisture protection)

Great, that’s exactly what I’ve been asking about all along...

The extension will surely have a roof, how will you connect it to the existing roof?

The roofer will take care of that, since the existing roof also has to be extended further (due to the still missing but to be retrofitted facade insulation). Therefore, I have no questions about the roof construction.

How do you avoid thermal bridges (connections between existing insulation / new insulation, etc... do you trust yourself to handle all that?

No, that’s why I only want the things I described here clarified/explained (which are the things I trust myself to do and so far don’t see the “magic” behind it...).

Thanks anyway.

....following a similar procedure. Even for an extension, you need structural analysis --> how deep, how wide, what reinforcement

How deep, how wide is already stated in the initial post. How reinforced? Precisely this is the purpose of this thread, answering my questions and pointing out if there is something I haven't mentioned that needs to be considered. It confuses me when an architect asks me as a layman how the reinforcement should be executed for something like this...

how thick is the base slab, what reinforcement for the base slab

Good question, what do you recommend?

what about the ring beam, how wide, how reinforced?

Ring beam? Regarding whether I should execute the foundation flush with the ground or above it and how moisture protection needs to be done? Do you mean the ring beam as the component that sits under the roof truss?

A ring beam must be constructed if:

[*]Buildings with more than 2 full stories (my house entrance is only one story)
[*]Buildings longer than 18 m (dimensions are in the initial post, 1.5m x 3.5m)
[*]Walls with many large openings (one door and one window 1.0m x 1.0m, should be within limits)
[*]If the soil conditions require it (no)

But as I said, first I need to know what my foundation should look like before worrying about the ring beam. Apart from the fact that according to the same source I myself could not decide this, but the necessity of a ring beam is determined by an architect and the size and reinforcement by a structural engineer.

Of course, one can say in general “Hey, simply connect the foundation to the existing foundation/wall, and the new walls to the existing walls with proper anchoring,” but what good does that do you?

You mean other than that some of my questions would already be answered?

Well, before I raise the walls, I would think about the complete wall structure

I already wrote that these considerations have long been made... The extension simply does not need to be integrated into an existing insulation, as there is none yet.
And the “before I start something, I think about the whole thing,” that’s exactly what I’m trying to do here. Exchange and information before I commission a service without plan or concept that after half the planning is then obsolete anyway.

No one can seriously answer that for you through the forum.

So how to connect the foundation to the existing basement wall or how to execute moisture protection and how high the foundation should rise above ground level can only be assessed on site? How is such planning supposed to take place? My basement wall is still significantly buried underground and I’m not going to excavate half my house to find out WHETHER I am realizing an extension... which also does not yet exist, but only my specifications or information about the desired result... so there I “see” (as far as my questions are concerned) the same as here, don’t I? It’s only about the foundation...
Should any information be missing, I am always happy to supplement it.

@BratacDD: THANK YOU! That already sounds like it will help me further
That I need an architect anyway before the construction project starts who plans everything ready for submission is out of the question, I have already spoken with the responsible authority about what is needed.
But as you rightly recognized, I want to first clarify my open questions regarding feasibility and manner.

Regarding your explanations about the foundation, I have a follow-up question: What does “stepped” mean?
I actually wanted to lay the foundation about 80cm deep (for frost protection), the basement foundation naturally lies much deeper (about 1.5m). So does my foundation for the extension also need to be laid at 1.5m depth?

Thank you!!!