ᐅ Single-family house converted for two families: experiences with floor-by-floor separation?

Hello everyone,

We are currently at the beginning of a construction project in Ludwigsburg (Sonnenberg South-West development area) and would like to gather early feedback on the choice of plot and the overall concept.

The plots are allocated through an application process (up to 3 preferences possible, selection not guaranteed). We currently have 4 possible plots shortlisted and need to prioritize them.

Development Plan / Restrictions

Plot size: approx. 440 – 525 m² (4730 – 5650 sq ft)
Slope: no (relatively flat)
Site coverage ratio: 0.4
Number of floors: 2 full stories
Building type: detached construction, single-family or semi-detached houses allowed
Roof style: flat roof (max. 3° pitch, specified)
Maximum height: approx. 9 m (30 ft) to parapet
Special conditions:
• Green roof required
• Photovoltaic system planned / can be integrated effectively
• No underground garage allowed

Homeowners’ Requirements

The plan is not for a conventional single-family house but:

A single-family house to be used by two families, separated by floors

(confirmed provisionally by phone and building regulations, final feedback pending)

Concept:
• Ground floor: approx. 140–160 m² (1500–1700 sq ft) + garden
• Upper floor: approx. 150–160 m² (1600–1700 sq ft)
• Goal: long-term use, not an investment
• rather modern, clear architecture (flat roof is mandatory anyway)

Households:
• two families (each early 40s, each with one child)

Additional requirements:
• preferably quiet location within the development area (no through traffic)
• good orientation (garden ideally facing south)
• practical access / parking solution (2 garages + 2 carports planned)

House Design / Current Status

Who is designing it:
Currently do-it-yourself / concept phase

What is still open:
• Construction method (prefabricated vs. solid, wood vs. concrete)
• Floor plans
• Basement (partial vs. full)

Our Questions to You

Which plots would you generally prefer in such a development area (quietness, location, orientation)?
From your experience, are there any “typical mistakes” in plot selection that only become apparent later?
How critical do you consider the concept of “two families in one single-family house separated by floors” from a practical point of view (sound insulation, everyday life, usage)?
What would you pay special attention to regarding the placement of garage/carport and access?

We are aware that we are still early in the planning and open to honest feedback.
The concept is still under discussion, especially regarding a suitable second family. If anyone is in a similar situation or wants more information, just search for “Sonnenberg South-West”; we are happy to exchange ideas.
Thank you very much!

Best regards
Richard

mmyellow schrieb:
A single-family house used by two families, separated by floors

By the way, that’s called a two-family house…

mmyellow schrieb:
How critical do you view the concept of “two families living in a single-family house separated by floors” from a practical point of view (soundproofing, daily life, use)?

These are typical issues in multi-family houses. Do you feel comfortable with someone living above you and possibly watching you sunbathe naked from upstairs? You really have to get along well. Noise can always be an issue. Who is allowed to use the garden?
And of course: What about the ownership structure? Who owns the house? Who finances it? Two condominiums? Rental? And so on.

I’m simply asking why and with whom you want to do this? With four parking spaces and a driveway, there is hardly any garden left.

P.S. I live in the lower unit of a two-family house and rent out the apartment upstairs. For me, it would be a no-go when building new (I bought the house as an existing property).

mmyellow schrieb:
Concept:
• Ground floor: approx. 140–160 m² (1500–1720 sq ft) + garden

mmyellow schrieb:
Floor area ratio (FAR): 0.4

mmyellow schrieb:
Plot size: approx. 440–525 m² (4700–5650 sq ft)

This can become very tight. On the largest plot, you are allowed to build just under 210 m² (2260 sq ft) of floor area.
I do not see anything in the development plan about Section 19 of the Land Use Ordinance, where the floor area ratio can be increased for access ways and parking spaces.
This means: you are allowed to build a total of 210 m² (2260 sq ft), including parking areas, on a 525 m² (5650 sq ft) plot. No more.

mmyellow schrieb:
Regarding a suitable second family,

There are very few who imagine their nest building in a single-family house sector only on the upper floor. Moreover, it is questionable how the plot use and division should be broken down in terms of costs.

I assume that, due to cost sharing, you want to split the plot with another party, but want to take most of the benefits yourself.

I don’t see the second builder. Usually, having an apartment on the upper floor only works well if you build the house and rent out the upper floor. Tenants tend to accept more in that situation. I would at least not invest money in a plot where I cannot take any barrier-free step or plant my own little tree.

mmyellow schrieb:
Which plots would you generally prefer in such a residential area (quietness, location, orientation)?
From your experience, are there “typical mistakes” in plot selection that only become apparent later?

Since you don’t know the dimensions yet, nor the heights or slope values, you should try to fit a 190 m² (2050 sq ft) rectangular template for the house. Mark a central stairwell on the front side. Additionally, add four templates each 3 x 6 m (10 x 20 ft) for parking spaces.
Altogether, that’s already 250 m² (2690 sq ft)… and then you’d probably need to also include a garden shed and similar structures. This far exceeds the 0.4 floor area ratio.
Also, consider the mandatory planting of several small trees. They are important on moist ground to retain rainwater.

Well, that probably won’t work. Please share a link to the zoning plan or planning permission.

mmyellow schrieb:
Plot size: approx. 440 – 525 m² (4740 – 5650 sq ft)
Slope: no (relatively flat)
Floor area ratio (FAR): 0.4

mmyellow schrieb:
Concept:
• Ground floor: approx. 140–160 m² (1500–1720 sq ft) + garden
• Upper floor: approx. 150–160 m² (1615–1720 sq ft)

Plot 440 m² (4740 sq ft) ==> FAR 0.4 = 176 m² (1895 sq ft)
Plot 525 m² (5650 sq ft) ==> FAR 0.4 = 210 m² (2260 sq ft)

160 m² (1720 sq ft) living area roughly equals about 180 m² (1940 sq ft) built-up area including garage, terrace, etc.

And the upper floor is certainly planned to include a nice balcony.

kbt09 schrieb:
Well, that probably won’t work. Please share a link to the development plan.

https://www.ludwigsburg.de/site/Ludwigsburg-Internet-2020/get/params_E-1944291867/25070918/Exposee_Sonnenberg_SW.pdf

https://www.ludwigsburg.de/site/Ludwigsburg-Internet-2020/get/params_E1942833911/25070939/Exposee%20Sonnenber%20SW_Anlage_1.1_Planteil_B-Plan.pdf

https://www.ludwigsburg.de/site/Ludwigsburg-Internet-2020/get/params_E201466591/25070940/Exposee%20Sonnenber%20SW_Anlage_1.2_Legende_B-Plan.pdf

https://www.ludwigsburg.de/site/Ludwigsburg-Internet-2020/get/params_E-1592578321/25070941/Exposee%20Sonnenberg%20SW_Anlage_2_Textteil_Festsetzungen%20B-Plan.pdf

mmyellow schrieb:
The plan is not for a traditional single-family house, but rather: A single-family house designed for use by two families, separated by floors

Please enlighten us why you don’t simply call this a “two-family house” – the conceptual difference is not yet clear to me.

mmyellow schrieb:
Concept: • Ground floor: approx. 140–160 m² (1506–1722 sq ft) + garden • Upper floor: approx. 150–160 m² (1615–1722 sq ft)

So the ground floor may be a bit smaller than the upper floor – does that mean there is a shared utility room on the ground floor?
How is a basement understood if underground parking garages are excluded?

mmyellow schrieb:
Rather modern, clear architecture (flat roof is predetermined anyway)

Am I reading here the common misconception called “Hornbach style”?

mmyellow schrieb:
Still open: • Construction method (prefab vs. solid build, wood vs. concrete) • Floor plans • Basement (partial vs. full)

A concept-neutral preliminary design is always a good approach. In my external series on basements, I also covered the topic of partial basements; see “Partial Basement: The solution between yes and no?”.

Who will be the second party? Do they already exist, and do you prefer the lower or the upper floor?

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