ᐅ Semi-detached house within a building plot measuring 8.5 m by 15 m (width x depth)
Created on: 20 May 2025 19:02
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GregorBerger
Dear housebuilding forum,
After several years of searching, we have finally purchased a plot of land (with an existing building to be demolished) in our desired location. This also marks the end of my many years of purely passive participation in this forum. Since we were primarily looking for renovation projects rather than new builds, we naturally have some initial questions.
The plot, approximately 500 m² (5,400 sq ft), lies within the scope of a development plan that was last updated 20 years ago.
Basic data about the plot and development plan:
The development plan places no restrictions on, among other things:
The plan is to build a semi-detached house with another family. Both families have two children each and require two home offices, resulting in a need for six rooms per semi-detached unit. If we make progress here, I will create another thread in the floor plan forum and fill out the questionnaire.
Since the building envelope width of 8.5 meters (28 ft) is too narrow for semi-detached units side by side, they would have to be arranged one behind the other. Garden access and terraces would then be located on the narrow sides (one facing the street and one facing the main garden at the rear). This type of semi-detached house is quite rare. I have looked around in real life but have not found any examples. Only the Büdenbender semi-detached house Gemello SD 135 roughly corresponds to this layout (though not to the measurements).
I have had some informal discussions with an architect I know (who now only works on office buildings) and the building authority, without encountering any fundamental contradictions so far.
I have already applied the @11ant basement rule, according to which a basement seems obligatory because there is more than 2 meters (7 ft) of height difference within the building envelope. However, I do not understand how this relates to the “base (plinth)” restriction of max. 60 cm (24 in) in the development plan. My layperson assumption would be that one floor (the ground floor? the lowest residential floor? the lowest full floor?) may start a maximum of 60 cm (24 in) above street level.
My first questions for you:
Thank you in advance,
Gregor
After several years of searching, we have finally purchased a plot of land (with an existing building to be demolished) in our desired location. This also marks the end of my many years of purely passive participation in this forum. Since we were primarily looking for renovation projects rather than new builds, we naturally have some initial questions.
The plot, approximately 500 m² (5,400 sq ft), lies within the scope of a development plan that was last updated 20 years ago.
Basic data about the plot and development plan:
- Building is permitted from 3 meters (10 ft) to 18 meters (59 ft) depth (so 15 m (49 ft) for the house)
- After deducting setback areas, a width of 8.5 meters (28 ft) is possible
- Slight slope across the building envelope with a rise of approximately 2.2 meters (7 ft)
- 3 full floors permitted
- Site occupancy index (ground coverage ratio) 0.4
- Floor area ratio 1.2 (cannot be fully utilized due to the aforementioned building envelope)
- Roof pitch 35–45°
- Knee wall (dormer wall) height 60 cm (24 in)
- Base (plinth) max. 60 cm (24 in) above the midpoint along the width of the access area, which is itself 30 cm (12 in) lower than the start of the building envelope
- General residential zone
- Open building style (detached buildings, no shared walls)
- Covered terraces allowed up to 1.5 meters (5 ft) outside the building boundary, provided the site occupancy index is not exceeded
- Roof indentations and structures permitted up to half the eaves length and at least 1 meter (3 ft) from the gable wall
- Fencing with native hedges. Along the street, an additional fence up to 1.2 meters (4 ft) high is permitted.
The development plan places no restrictions on, among other things:
- Building type
- Number of residential units per building
- Height limits
- Basements
- Parking spaces
The plan is to build a semi-detached house with another family. Both families have two children each and require two home offices, resulting in a need for six rooms per semi-detached unit. If we make progress here, I will create another thread in the floor plan forum and fill out the questionnaire.
Since the building envelope width of 8.5 meters (28 ft) is too narrow for semi-detached units side by side, they would have to be arranged one behind the other. Garden access and terraces would then be located on the narrow sides (one facing the street and one facing the main garden at the rear). This type of semi-detached house is quite rare. I have looked around in real life but have not found any examples. Only the Büdenbender semi-detached house Gemello SD 135 roughly corresponds to this layout (though not to the measurements).
I have had some informal discussions with an architect I know (who now only works on office buildings) and the building authority, without encountering any fundamental contradictions so far.
I have already applied the @11ant basement rule, according to which a basement seems obligatory because there is more than 2 meters (7 ft) of height difference within the building envelope. However, I do not understand how this relates to the “base (plinth)” restriction of max. 60 cm (24 in) in the development plan. My layperson assumption would be that one floor (the ground floor? the lowest residential floor? the lowest full floor?) may start a maximum of 60 cm (24 in) above street level.
My first questions for you:
- Are you familiar with similar houses, possibly with names for Googling or similar?
- Would you approach this topic differently?
- What does the rule about the base (plinth) mean?
Thank you in advance,
Gregor
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GregorBerger22 May 2025 09:21hanghaus2023 schrieb:
Could you please provide a height profile for the entire plot? Not just the building area.Attached. I’m quite surprised myself how accurate the data is. The steep rise on the far right of the profile shows the retaining walls leading to the higher neighboring property at the back. G
GregorBerger22 May 2025 09:43Hello again,
maybe it gets tedious to say thank you every time. But I am genuinely impressed! In some other forums, there is the option to “buy someone a coffee.” It’s a pity this isn’t possible here. How can one show appreciation?
That would still be possible with a “split-level floor” on the ground at street level, which only protrudes about 1.4m (4.6 feet) on average and therefore doesn’t count as a full floor, right?
The legend of the site development plan also indicates a building line (with a different dotted pattern), but this is not applied in our planning area. Both the front and rear boundaries are building limits.
However, I did not ask about a subdivision strictly parallel to the street, but about a flag lot with its own 3m (10 feet) wide access. At that moment, the previously positive mood in the discussion suddenly changed. I guess I stirred up a hornet’s nest.
That is also a fallback option. Of course, it comes with some disadvantages. For example, I am thinking of:
None of these points are deal-breakers. We expect some compromises.
I will reply separately to @ypg.
maybe it gets tedious to say thank you every time. But I am genuinely impressed! In some other forums, there is the option to “buy someone a coffee.” It’s a pity this isn’t possible here. How can one show appreciation?
11ant schrieb:
Without limiting the base, there would be a strong temptation to raise the ground floor on the garden side to the level of the surrounding terrain, which could result in an increased building height.
That would still be possible with a “split-level floor” on the ground at street level, which only protrudes about 1.4m (4.6 feet) on average and therefore doesn’t count as a full floor, right?
11ant schrieb:
Without making the front edge of the building window the building line, I don’t see any legal basis for such an extension obligation. The reason for rejecting the subdivision is that the rear part would then not be legally “developed” and thus would have no prospect of building permission (unless the front neighbour granted the relevant plot access rights).
The legend of the site development plan also indicates a building line (with a different dotted pattern), but this is not applied in our planning area. Both the front and rear boundaries are building limits.
However, I did not ask about a subdivision strictly parallel to the street, but about a flag lot with its own 3m (10 feet) wide access. At that moment, the previously positive mood in the discussion suddenly changed. I guess I stirred up a hornet’s nest.
11ant schrieb:
However, I advise you not to go through the hassle, but instead build a classic two-family house with separate floors (even if the upper apartment is a maisonette).
That is also a fallback option. Of course, it comes with some disadvantages. For example, I am thinking of:
- Sound insulation between floors can never be as good as between two semi-detached units with separate walls
- Garden separation and mutual privacy of gardens are not possible in a two-family or multi-family house
- Both units would have to be maisonettes; otherwise, the living space would be limited to about 110m² (1180 sq ft)
- If one apartment were ever subdivided, the two-family house would become a multi-family dwelling, with all the implications for building regulations, parking spaces, etc.
None of these points are deal-breakers. We expect some compromises.
I will reply separately to @ypg.
H
hanghaus202322 May 2025 09:55I have adjusted this a bit to reflect reality.
I would position the first entrance 20cm (8 inches) above the street level and the second one 60cm (24 inches) higher.
In my opinion, four floors are too many.
I would position the first entrance 20cm (8 inches) above the street level and the second one 60cm (24 inches) higher.
In my opinion, four floors are too many.
@ypg already calculated the living area without room partitions, etc., per floor to be about 53 sqm (570 sq ft), which, based on experience, results in approximately 45 to 48 sqm (485 to 515 sq ft) of usable living space including the staircase per floor. (It’s important to understand that a 3 m (10 ft) wall with a thickness of 15 cm (6 inches) uses about 0.45 sqm (5 sq ft) of space).
So, 2 floors would be around 90–96 sqm (970–1030 sq ft) plus the attic, which is the outcome of @hanghaus2023’s study in post 33, and technical rooms still need to be accommodated.
Just to give a clear idea of the dimensions involved.
So, 2 floors would be around 90–96 sqm (970–1030 sq ft) plus the attic, which is the outcome of @hanghaus2023’s study in post 33, and technical rooms still need to be accommodated.
Just to give a clear idea of the dimensions involved.
H
hanghaus202322 May 2025 10:54I have already sketched the ground floor and first floor. The parents' bedroom is in the attic studio.
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GregorBerger22 May 2025 10:56ypg schrieb:
There is certainly a good reason why you haven’t found what you’re looking for with these houses: your space requirements. Well, there are quite a few houses with under 50 sqm (5 feet) of living space on the ground floor and 6 rooms. The difference is that they aren’t square and usually have more than just two floors like the typical "urban villa" style.
ypg schrieb:
Yes, but at some point, you have to break the mental loop and consider whether a huge effort with many compromises is really worth it to achieve something that no longer matches the original wish. That’s true, but our wishes are flexible. The starting point was that the plot allows for 127 sqm (1365 square feet) footprint and 3 full floors (+ attic + possibly a basement on a slope). That results in something between 400 and 500 sqm (4300–5400 square feet) of total floor area – which is definitely more than enough for two families with a total of four home offices.
ypg schrieb:
I don’t assume you’re planning to give part of the plot away to your friends out of kindness, so this must be a financial issue. How much was the plot? Half a million. And yes, exactly. You want to make good use of this centrally located, wonderfully situated plot. Not down to the last square centimeter, but certainly more than just for a 6-room single-family house. The neighbors have done the same, but not as a semi-detached house—they went for duplex or multifamily houses instead.
ypg schrieb:
And how much of that cost is accepted by the building partners? One party will definitely get the short end of the stick here. ypg schrieb:
It’s becoming clear: the rear part will lack a floor that the front part has. That’s correct, but it’s not a dealbreaker. There is no requirement for the two halves of the semi-detached house to be completely identical.
ypg schrieb:
And on these footprint dimensions, I don’t see your stated room program fitting. So you’d necessarily have to build upwards. That’s okay. I’m no enemy of stairs. The idea is also to have the home office on its own level, as far away as possible from the ground floor bustle.
ypg schrieb:
Then you’ll just barely fit your room program. But I warn you: no one should come back saying that a straight staircase, a pantry, or a walk-in closet have always been a dream and that you want to realize one or two of those wishes. None of those are our wishes. Those would have probably come up in the floor plan form, but we’re not at that stage yet…
ypg schrieb:
So, what about the building partners? Don’t they have any room program? They would have one floor less!
You can of course add a creative third floor and then an attic. I believe that choice was open? Roof pitch 35–45°, ridge direction flexible, dormer or inset up to 50% of the width.
ypg schrieb:
The question I have is: what were you thinking when you bought the plot? I mean, there probably was no planned semi-detached house yet? Yes, there was. With a fallback option for a two-family house, a multi-family house, or a single-family house. The single-family would push our budget to the limit. The bank is willing to lend more money for a multi-family house because they factor in hypothetical rental income and tax benefits.
ypg schrieb:
Also, in my opinion, the differences and disadvantages for the front unit are significant and not something you want to live with for the next 20 years. The division to the rear unit is very large. The “little guy” will be watching as the person in the upper unit passes by their house and 40 sqm (430 square feet) garden on their way to their own 200 sqm (2150 square feet) space. Beyond that, there’s an implied arrangement that you basically have to pay for everything for the other (slope maintenance, retaining walls, etc.). No friendship can survive that, mark my words! You’re correct about the situation, but that has been communicated and agreed with the building partners from the start. They can, however, withdraw with less pain (unlike us); we bought the plot alone first.
ypg schrieb:
My suggestion: place a nice two-story house there. Use the garden on the upper floor for yourselves. That way, you also have space downstairs for a bike storage. You could possibly plan a granny flat. But you would have to borrow expensive money for the granny flat, which might not pay off. Here are the two residential units of a duplex with their ground floors and garden shares. That looks very interesting, but I don’t fully understand it yet. Two stories plus basement on a slope + upper floor (garden level) + second upper floor? Flat roofs are not allowed, so is there an attic on top? Where is the separation line between the two residences?
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