Planning a single-family house with 4 children's bedrooms

Hello dear forum,

we are a family with currently 3 children, a 4th on the way, and want to build a house in Brandenburg (near Berlin).
We are absolute beginners in the field of house building and cannot decide on many things simply because we have no deeper knowledge.
Maybe you can help us or give us a few tips, we would be very grateful.

Now to the basic data:

We have been promised the plot of land, the notary appointment will probably take place in 2 months.

The plot is about 1000 sqm and has a floor area ratio of 0.2 according to the development plan. One full storey may be built, deviating from this, an additional full storey is allowed if it is a full storey built in an attic space.
Gable, hip or half-hip roof is mandatory. 30°-50° pitch.

Our "dream floor plan" that we have thought out is attached. The floor area would be 12.85 x 9.89 with a gable roof on top. Knee wall at 1.80.

A city villa is unfortunately not possible due to the above restriction, as the roof would not be integrated into the second storey.

What is important to us:
Living area should not be less than 160 sqm, ideally around 180-190.
At least 3 children's rooms should be available.
A study must also be available (on the ground floor).
A knee wall of at least 1.80 m would be important, as we have very tall men in the family.
Windows on all four sides on the upper floor (floor-to-ceiling not necessary)
A second bathroom upstairs. (Sanitary and tiles/floor coverings we would get at purchase price.)

A budget of €450,000 without ancillary construction costs is planned for the house.

Since this is a very tight budget for such a house, our question now is: is it even possible?
What can be omitted to save money?

Basically, we see only two possibilities. Either our floor plan and wishes and cutting a lot (insulation, ventilation system, etc...)
Or simply a smaller prefabricated house as specified, according to KfW standard. What would be the advantages or disadvantages here?
A few questions on that:

Should one build with a prefab house company or rather commission the trades oneself, what is cheaper or better?
Since we have some extra wishes (floor plan, knee wall, additional windows) prices with prefabricated house companies quickly became noticeably more expensive. In addition, they include a lot that we might want to omit to fulfill our wishes (see below regarding heating, insulation and co)?

We have already spoken with several house building companies and are now completely clueless, as each considers their construction method to be the best.
Massive construction or timber frame? If massive, which wall thickness? 24, 36?

Styrofoam insulation, lime or possibly no insulation at all?
What is more susceptible to mold/algae? What costs the least?

Centralized/decentralized ventilation system or rather none at all (I like to ventilate very much)?
I tend to none, my husband tends to a ventilation system.

My purely subjective and absolutely non-expert opinion: I fear that the ventilation noise might annoy me, the air might become too dry and also too "clean," and the pipes after years of use might eventually develop mold or break due to wear and tear and then you have to tear everything open and replace it.
Furthermore, it is also a not insignificant cost factor that would save us a lot of money if we omit it.

My husband says the technology has existed for a long time, nothing will mold or break, on the contrary, the house would certainly be mold-free and you would always have perfectly filtered air, humidity, and temperature.

What would be your professional opinion on this?

Then we tend toward underfloor heating because I always freeze without it, so air/water heat pump and not air/air.

Can one save on the floor plan? Should one draw as many straight walls as possible and avoid corners to make it cheaper?

Does the house become cheaper if you only aim for a 30° roof pitch and not e.g. 45°?

Should one build according to KfW standard, can one save by building without it?

We would only apply for KfW funding if we build in timber frame construction with a prefab company, as the additional costs for the analyses for solid houses would unfortunately be too high due to our wishes for the house.

We would also have liked to use the ILB funding, but our plot is just outside the area boundary.
Now two companies told us that then you have no chance and two others said that that would be no problem and the architect could simply apply for an area boundary extension. It would just be a letter and would only still have to be signed by the city, but would not be a problem.
Are some too lazy to submit applications or are others exaggerating?

So many questions, hopefully half understandable?
Many thanks in advance and have a nice evening!

A knee wall of at least 1.80m would be important, as we have very tall men in the family.

Let's start from the beginning: what is the development plan like? What is permitted? What are the maximum heights? Because the knee wall and roof pitch are planned based on that. Men also don't have to walk along the exterior wall. If they want to do that, I recommend a rubber cell. Otherwise, storage space and cabinets can be well accommodated under roof pitches.

What can be omitted to save money?

Oh, a lot of things that you do want, though.

We have already spoken with several house construction companies and are now completely at a loss, as each considers their own building method the best.

Honestly? You have no choice. And in the end, it does not matter how living space is created. For you, the number at the bottom line counts.

Styrofoam insulation, lime or possibly no insulation at all? What is more susceptible to mold/algae? What costs the least? Central/decentral ventilation system or better none at all (I really like to ventilate)? I tend to none at all, my husband tends toward the ventilation system.

Since we have some special requests

450,000€ divided by 3,000€/sqm results in 150sqm without much frills and special equipment. Period. With gained living space in the attic as well as own work, considerably more is possible.

Can you save on the floor plan?

Yes, by foregoing elaborate structural engineering, a second bathroom, by using the attic, hallway area, etc.

My husband says

He is right, too. He talks about comfort. But comfort also has to be paid for. I am always pro controlled residential ventilation, but it costs! And if you are tied to the living space anyway because of 4 children, then you probably have to do without it because it does not fit the budget.

Will the house be cheaper if you only aim for the 30° roof pitch and not for example 45°?

No, that results from the need for living space, how much knee wall is ultimately possible respectively how much ground floor area and roof space is sensible. Such a thing cannot be answered in general. How much living area does your house design have?

The plot is about 1000 sqm in size

I think that's quite large, especially if the budget is a bit tight. What do you want to do with the large plot and how do you plan to maintain it?

Good morning,
Thank you very much for the quick responses!

Let's start from the beginning: what is the development plan like? What is allowed? What are the maximum heights?
Because after that you plan knee wall and roof pitch.
Also, men don’t have to walk along the outside wall. If they want to do that, I advise a padded cell. Otherwise, you can nicely use storage space and cabinets under roof slopes.

According to the development plan, the plot has a floor area ratio of 0.2. Building is allowed with one full storey, deviating from this, an additional full storey is permitted if it is a full storey built into an attic space.
Gable, hip, or clipped hip roof is mandatory. Roof pitch 30°-50°.
There are no maximum height restrictions.

How much living space does your house design have?

185 sqm

I find that pretty big, especially if the budget is a bit tight. What do you want to do with the large plot and how do you want to maintain it?

That’s true, but it was actually the best offer (the older gentleman is now under time pressure and has therefore lowered the price significantly). We originally wanted a plot of about 600 sqm around the corner, but that is supposed to cost the same as the 1000 sqm, so we decided on the larger one. Unfortunately, splitting it is not possible, we have already inquired about that.

Basically, we see only three options:
Either we build with a cheap prefab house provider with hardly any compromises on our wishes.
We were advised against that because supposedly Styrofoam in the walls is not a good choice and also the heating system they install would be of lower quality?

Or we build a smaller house with a somewhat more expensive prefab house provider.
With allegedly better vapor-permeable construction and supposedly better heating?

Or we build a bigger house with a local solid construction company.
With 36 cm blocks, but without insulation and without ventilation system.

Or we handle all the trades ourselves, if that gets the best price-performance ratio?

What would be best in the long run?

Or we build a smaller house with a slightly more expensive prefabricated house provider.

I would definitely prefer a smaller house. They are often cozier, more homely, and avoid (modern) gimmicks. Things like a kitchen island, L-sofas for giants, straight staircases, garages attached to the house (often with direct access to the house), mega large terraces (with large, gray tiles). Stuff like that. Not needed.

I don't live in Germany and here houses in urban areas tend to be rather small. Six people comfortably live on 140-160 sqm here. No problem. However, nobody has a straight staircase in the middle of the house either.
I like the houses here because they look lived-in and not "hip."

A long kitchen counter with a cozy table in the room instead of an island in the room, I still find great.

You could even build small and cheap. That option is still available.

We were advised against that because supposedly Styrofoam in the walls isn’t a good choice and also the heating they install would lose quality?

Styrofoam in the walls? Usually more like sand-lime brick as load-bearing masonry and in front of it an external thermal insulation composite system (ETICS) based on EPS (Styrofoam). That is common building standard. It has advantages (cheap, quick to build, thermal bridges are easy to control, thermal insulation and static function of the exterior wall are separated) and disadvantages (north side tends to algal growth because EPS hardly retains heat and cools down quickly at night).

Which heating system? An exhaust air heat pump from Proxon / Zimmermann? -> Better to avoid, my personal opinion.
With an air-to-water heat pump, I wouldn’t insist on Viessmann / Vaillant / Bosch aka German manufacturers, other companies also have good technology and are often more affordable, depends on the specific case.

in return with the supposedly better diffusion-open construction and a supposedly better heating system?

Diffusion-open construction and breathable walls are marketing nonsense in 99% of cases in modern building.
Either consistently: clay plaster on timber frame with straw/flax/wood fiber insulation and a ventilated exterior facade (all more expensive than standard) or ignore it. Your house “breathes” through open windows or a ventilation system, not through the wall.

KfW40 is, bottom line, in most cases more expensive to build than the GEG2024 standard. Off-the-shelf in a catalog house it can bring a financial advantage, but as a single object the additional costs for planning and certification are usually too high.