ᐅ Semi-detached house without a basement next to a semi-detached house with a basement

Becca_K schrieb:

My husband and I unexpectedly received an offer for a plot for a semi-detached house in a local residents’ development model near Munich. The second half of the semi-detached house was already allocated some time ago; we have already met our potential neighbor: his building permit / planning permission has been approved, and he is just waiting to start construction.

A development plan / zoning plan is something different; it is a legal framework provided by the municipality that regulates and coordinates the development of an entire area. What you mean is a building permit – so is he not building with an individual permit instead?

Becca_K schrieb:

In our case, we want to avoid having a basement, not only for financial reasons, while he will definitely build one. Also, he is building with solid masonry, whereas we plan to build a turnkey prefabricated wooden house.

Choosing to forgo a basement is rarely just a matter of preference because the plot of land and especially its topography play a significant role in whether it is economically sensible. The "11ant basement rule" roughly says that if there is a height difference of two meters (about 6.5 feet) within the house’s footprint, avoiding a basement can cost about the same as building one. You can also estimate proportionally: for example, a 60cm (2 feet) height difference means the cost saved by avoiding a basement corresponds to about 30% of the basement construction costs.

What are your motives for preferring a "prefabricated wooden house"? Do you mean the widely used timber frame panel construction, often mistakenly called "timber stud construction"? And how much expertise or knowledge underlies this preference? I usually recommend clients keep their choice of construction method open at least until the early planning phase of the housebuilding process. From my experience, there is no fundamentally right or wrong choice between wooden and masonry construction; an individual evaluation is always worthwhile.

Becca_K schrieb:

Where on these 300 pages would I find the information relevant to us now?

The thread is certainly worth reading, even though many posts necessarily get bogged down a bit in the back-and-forth between the mayor and the neighbor over old and new middle-class houses. The most important reassurance for you is that the biggest potential issue has already been defused: the party building the basement starts construction first. So you, the party without a basement, can relax on that front. Another critical point in uncoordinated semi-detached house construction—which, as far as I recall, isn’t thoroughly discussed in the Goalkeeper thread but I have raised in all relevant threads—is the alignment of the two halves at the joint "house profiles at the shared wall side." Regardless of possible different construction methods, ask your neighbor about their architect.

You can find my collected comments on this topic mainly by using the forum search function with the keyword "semi-detached house" and filtering by "written by: 11ant", as well as by searching for "without basement" or "with basement." Also, externally (searchable with quotation marks) under "A semi-detached house has TWO halves." You can also find several contributions about soundproofing in terrace houses through the forum’s search function.

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@11ant: Many thanks for your numerous tips and advice! I’m still not fully confident with the technical vocabulary, that’s true. ;-) I didn’t mean the zoning plan but actually the building permit, sorry. He has apparently already submitted it, and it was approved by the local authority, so now we have to adapt to it (due to the profile uniformity required by the zoning plan).

The plot is “flat as a board,” as they say here, or also: “Oh, great, a beginner’s plot!” (quote from my brother-in-law). A soil survey is already available, and the house supplier is familiar with the development area (they are apparently building about 10 other houses there simultaneously) and described it as uncomplicated. Therefore, I suspect that – following your rule – a basement is probably not financially worthwhile for us.

The reason we are building with timber frame panel construction ;-) is simply because we like the offer from this builder and we want/need to build a KfW 40+ energy-efficient house (due to the KfW subsidy). A competitor told us that this would not be possible with bricks.

Unfortunately, the neighbor’s architect is “a family friend who is already retired,” otherwise we would of course have tried to involve him as well. I’m also worried about how the house sides will fit together, but I guess I will have to rely at least partly on our house builder...? What else could I do?

Becca_K schrieb:

The house provider already knows the building site well (he is reportedly constructing about 10 other houses nearby at the same time) and described it as straightforward. [...] I’m also worried about how well the house sides will fit together, but I guess I have to at least partly rely on our construction company for that...?

Here, the fit of the house profiles worries me the most when building a mixed-gender semi-detached house. The provider’s experience with ten other houses nearby—which probably don’t need to fit together in the same configuration—is more concerning than reassuring. My concern is not just about the general visual similarity that the municipality looks for (where differences of three or even five centimeters (1 to 2 inches) don’t matter), but about the airtightness of the component joints. Water, especially, finds its way in at places that can be extremely difficult to protect. On-site masonry construction allows for much more flexible adjustments than prefabrication. Aiming for millimeter-precision can still result in noticeable gaps in reality.

Becca_K schrieb:

Therefore, following your rule, I suspect that a basement is not really financially worthwhile for us.

Yes, Brettleben, that’s exactly right. Especially because townhouses typically offer plenty of “basement replacement space” potential in the attic.

Becca_K schrieb:

The fact that we are building with a timber frame panel system ;-) is only because we like the offer from this builder and need/want to build a KfW 40+ house (due to KfW funding). A competitor told us this wouldn’t be possible for him with bricks. [...] What else could I do?

You could forego that reversed prioritization between tail and dog. Achieving an efficiency house 40 standard is indeed more expensive with masonry than with timber frame panels and their almost inherent integral thermal insulation system. However, this highest energy regulation standard is economically negligible since the KfW subsidies roughly offset the extra costs. Moreover, especially with a profile-matched end townhouse (which is basically no different from a semi-detached house once you exclude a middle unit), you benefit from full insulation coverage on the largest exposed facade. That means the payback period from energy savings will easily exceed the expected holding period of the property. Therefore, you can confidently drop this extra energy regulation overachievement and gain the advantage of a construction method that is easier to adapt to the neighboring half. My recommendation is basically “set in stone” here: choose the same roofer for both halves.

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We had exactly your situation within our circle of acquaintances. From the reports, I can tell you the following: build together.

It’s not just the basement but also all the connection points that are affected. First of all, it looks terrible when the two halves don’t match (roof eaves, window sizes, house color), but the purely technical process is a disaster.

- Scaffolding for one side interferes with the other
- The insulation on one half must be installed before the other connects, but insulation cannot stay exposed to wind and weather indefinitely and will deteriorate. Insulation thicknesses must be agreed on in advance.
- The connection points on the roof are complicated.

The end result is that the semi-detached house now has an offset of 10 cm (4 inches). Naturally, it looks very high-end. Not to mention the style.

But basically, the side with the basement MUST always be built first. Supporting it otherwise would cost a five-figure amount.

I can only fully agree with the post by @Einplan (based on four decades of residential building planning experience). In theory, this is always my approach as well. Unfortunately, in practice, there are limitations that prevent implementing the pure theory exactly as intended—usually in cases like this because the neighbor, for financial reasons, does not have the patience to wait for their duplex partner.

Nevertheless, I would strive to come as close as possible to the ideal of synchronizing your own foundation slab timing with the neighbor’s basement ceiling, and ideally using the same structural contractor and, as already mentioned, the same roofer (using the same carpenter would be even better). It is also highly recommended to coordinate the finished floor heights of the structural slabs.

If the neighbor’s architect is really retired, they have most likely canceled their professional liability insurance and effectively given up their authorization to submit plans, which means a third party will stamp the approval. If the neighbor is the architect themselves, they would be the ideal coordinator for the entire project. Otherwise, I would be happy to offer my own involvement.

In all interior finishing trades, you would have full freedom to assign and select specifications differently from the neighbor.

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