ᐅ Foundation for an Extension: Is This the Right Approach? Any Experiences?

Hello.
This is my first post here, so I’ll include a brief introduction.
My name is Stefan, I’m 31 years old, and I recently bought a house built in 1951. I am currently renovating and remodeling it.
Although I’m not a professional (trained as a business administrator, not a tradesperson), I’m doing a lot of the work myself during the renovation.
Right now, I’m gathering information for a planned extension at the front of the house (the main entrance will be enlarged / extended outward by 1.5m (5 feet) and the front door will be rotated 90°).

I’ll describe how I have planned this construction project so far and would appreciate advice on whether this is correct or how it might be better / more practical / feasible at all.
As I mentioned, I’m not a professional, so please excuse any missing technical terms or often amateurish expressions.

The extension itself is planned to be 1.5m (5 feet) deep and 3.5m (12 feet) wide, with the front door located on the short side at a 90° angle to the usual front of the house.
The main entrance (now and in the future) is about 50cm (20 inches) above ground level and is reached by 3 steps.

The structure will be built with aerated concrete blocks or Poroton blocks, 17.5cm (7 inches) thick.
For the foundation, a trench 80cm (31 inches) deep will be dug around the outside and filled with concrete to ensure frost protection.

Here is my first specific question: Is it sufficient to build this foundation up to ground level and then build the walls on top (with appropriate moisture protection), or should the foundation be cast significantly above ground level using formwork? Possibly even the full 50cm (20 inches) that will need to be backfilled later?

How should the connection to the existing house and basement wall be made? Should it simply be concreted together (“cast in place”) or is a moisture barrier required between the basement wall and the foundation for the extension?
Of course, it is clear that insulation is necessary at the transition to the outside (toward the soil).

The 50cm (20 inches) above ground to match the house entrance level will be filled with construction rubble. Above this, screed, underfloor heating, top screed, and tiles will be installed (the same continuous flooring as in the adjacent hallway extended by the new extension). Any objections?

I hope I have explained my project clearly, and even if my planning is misguided, I kindly ask for well-intentioned advice.

Thanks in advance and best regards

Hello,

Huti schrieb:

I hope I was able to explain my project clearly, and even if my planning is misguided, I would appreciate well-intentioned advice.

Get a professional to design the extension for you. Detailed planning in a forum is not advisable. This extension, especially with the connection to the existing building, needs to be properly planned.

Regards

Hmm, I actually thought my questions could be reasonably answered with some technical knowledge. It should be fairly general whether, for example, the existing foundation needs to be separated from the new one, or if you simply pour such a foundation 10cm (4 inches) above ground level, or preferably use formwork and build the full 50cm (20 inches) height, or even avoid extending above the soil altogether and instead apply a moisture barrier to the exterior of the masonry...

I don’t quite understand what the professional on site absolutely needs to inspect or what you mean by saying it has to be planned...

But thanks anyway for your contribution.

Huti schrieb:
It should be relatively universally applicable,

NOTHING is universally applicable when it comes to renovating existing buildings (and extensions are included in this).

Huti schrieb:
for example, whether the existing foundation needs to be isolated from the new one...

What do you mean by “isolated”?

Huti schrieb:
I don’t quite understand what the expert on site MUST inspect or what you mean by it needs to be planned...

Well, the foundation has to be stable (structural integrity), the energy-saving regulations must be met (thermal insulation), and waterproofing must be properly designed and executed (moisture protection). The extension will presumably have a roof—how do you plan to connect it to the existing roof? How will you avoid thermal bridges (connections between existing and new insulation, and so on)? Do you feel confident handling all of this yourself?

Best regards

There are no universal rules when building onto an existing structure (and this includes extensions).

I had been thinking from the perspective of other trades. For example, a tiler uses tile adhesive or mortar to lay tiles. A mechanic always uses the same tools when changing brakes on a car. So when it comes to masonry, I assumed that a mason would also follow a similar procedure every time when laying a foundation.

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 cast concrete onto it, or is a damp-proof membrane required between the basement wall and the new foundation for the extension?”

Well, the foundation must be stable (structural engineering).

That is why I described how it is going to be constructed and what loads (size, material, wall thickness) will be applied. Here, I return to my apparent misconception that such information should be enough for a professional to give an opinion, at least if my approach will not work and why.

The energy saving regulations must be followed (thermal insulation).

The building envelope will be fully insulated; there is currently no insulation and it will be installed completely after the extension is finished. That is why I have no questions concerning the required thermal insulation.

Waterproofing must be properly planned and executed (moisture protection).

Great, that is exactly what I have been asking about the whole time…

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

The roofer will take care of that, as the existing roof needs to be extended as well (due to the façade insulation that is not yet installed but will be retrofitted). So I have no questions about the roof construction.

How will you avoid thermal bridges (connections between existing and new insulation, etc.) … do you feel confident handling all that yourself?

No, that is why I only want to clarify or have explained the points I mentioned here (which are the things I feel confident about in execution and so far do not see any “magic” behind them…).

Thanks anyway.

....using a similar approach. Even for an extension, you need structural engineering --> how deep, how wide, what reinforcement

How deep and how wide are already stated in the initial post. How to reinforce it? That is exactly the purpose of this thread: to answer my questions and to point out if there is anything I didn’t mention that needs to be considered. It confuses me when an architect asks me, as a layperson, how reinforcement should be done for something like this...

how thick is the slab, what reinforcement for the slab

Good question, what do you recommend?

what about ring beam, how wide, how reinforced?

Ring beam? Regarding whether I should build the foundation level with the ground or higher, and how to do moisture protection? Don’t you mean the structural element that sits under the roof frame when you say ring beam?

A ring beam must be constructed in the following cases:

• Buildings with more than 2 full stories (my house entrance will be single-story only)
• Buildings longer than 18 m (dimensions are in the initial post, 1.5 m x 3.5 m (5 feet x 11.5 feet))
• Walls with many large openings (one door and one window 1.0 m x 1.0 m (3.3 feet x 3.3 feet), should be fine)
• If the soil conditions require it (no)

But as I said, first I need to know what my foundation should look like before I worry about a ring beam. Besides, according to the same source, I wouldn’t even decide that myself; the necessity of a ring beam is determined by an architect, and the size and reinforcement by a structural engineer.

Of course, you can generally say “Hey, just connect the new foundation to the existing foundation/wall, and anchor the new walls to the existing ones accordingly,” but what good would that do you?

You mean besides the fact that it would already answer some of my questions?

Before I raise the walls, I would think about the entire wall structure

I already wrote that these considerations have long been made... The extension simply doesn’t need to be integrated into existing insulation because it doesn’t exist yet.
And the idea of “Before I start anything, I consider the entire project” — that is exactly what I am trying to do here: exchange and gather information before commissioning a service that will become obsolete halfway through planning anyway.

No one on the forum can give you a reliable answer.

So the question of how to connect the foundation to the existing basement wall, how to implement moisture protection, and how high the foundation should protrude above ground level can only be assessed on site? How is such planning supposed to take place then? My basement wall is still well below ground. I’m not going to excavate half my house to find out WHETHER I’m doing an extension—which doesn’t even exist yet, by the way, it’s only my specifications and desired outcome… So in terms of my question, isn’t my “view” essentially the same as it is here? It’s only about the foundation...
If there are any missing details, I’m happy to provide more.

@BratacDD: THANK YOU! That already sounds like it might get me further.
That I will need an architect anyway before starting the project to create approval-ready plans is certain; I have already talked with the responsible authority about what is required.
But as you correctly recognized, I first want to clarify my open questions regarding feasibility and method.

I have one follow-up question about your explanation of the foundation: what does “stepped” mean?
I planned to set the foundation about 80 cm deep (31 inches) for frost protection; the basement foundation is of course much deeper (about 1.5 m (4.9 feet)). So does that mean my foundation for the extension must also be set 1.5 m deep?

Thank you!!!