Construction project feasible with financial resources?

Hello everyone,

below are our details and the question to your collective knowledge & experience: is it realistic to put our project into practice or should we rather leave it alone?

General information about you:

[*]Who are you? Married couple
[*]How old are you? 34
[*]Do you have children? No
[*]Are children planned? No
[*]What do you do professionally? Public service employee & marketing
[*]We are employees//from 2026 additionally part-time self-employed
[*]How many hours do you work? 39h//from 2026 prospectively about 3-5h more for me

Income and asset situation:

[*]What income do you have (gross/net)? Net 5350 € (excluding income from part-time self-employment, as this is a wish and not yet a fact)
[*]How much equity do you have? 150,000 €
[*]How much equity do you want to invest in the house project? 120,000 €

Expense situation:

Housing costs, internet & mobile phone 322 €
We currently live in the attic apartment of my parents-in-law and only pay a flat rate for additional costs, which is relatively "high" due to electric heating.

[*]current cold rent 0€
[*]current warm rent 267 €
[*]phone, internet, mobile 55 €

Mobility costs 629 €

[*]629 € for two cars, of which 300 € is a private loan until August 2026
[*]One car is to be sold with the new place of residence.

Insurance costs 253 €

Living expenses 809 €

[*]Groceries 400 €
[*]Restaurant costs 70 €
[*]Care/drugstore 150 €
[*]Medications 10 €
[*]Clothing 40 €
[*]Club fees/gym 70 €
[*]Streaming services 28 € (streaming services)
[*]Tickets 10€
[*]Donations 31 €

Savings: 2,230 €

[*]Vacation 200 €
[*]House 1,500 €
[*]Retirement provision 500 € ETFs
[*]Hobbies/gifts 30 €

Income and expense totals:

[*]Total income 5,350 €
[*]Total expenses 4,684 €
[*]Balance 1,766 €
[*]of which sum cold rent and dispensable savings (e.g. savings rate for house) 1,200 € savings + 100 € vacation + 450 € car + 70 € gym
[*]Available balance 3,027 €

General information about the property:

[*]How large is the plot? 494 sqm
[*]What are its dimensions? 21.73x29.47x19.59x19.38
[*]What is the standard land value? 310 €/sqm
[*]New build
[*]Garages? Single garage or carport as own work
[*]How large is the house? (living area / usable area) 115 sqm living area planned for us plus ELW approx. 40 sqm in the attic
[*]What is the market value of the plot and house after completion? No idea

Construction costs:

[*]Plot costs 153,000 €
[*]Development costs Already developed
[*]Additional acquisition costs 2,300 € notary & land register + 7,500 € property transfer tax & 1,000 € registration survey office = 10,800 €
[*]Incidental construction costs (e.g. house connections, soil expert, construction electricity, etc.) 11,000 €
[*]Outdoor facilities/terrace, paths, garden design, fences, etc. 10,000 €
[*]Financing costs (e.g. fees or commitment interest)
[*]Total costs 195,600 €

Other costs:

[*]Kitchen costs 30,000 €
[*]Furniture, lamps, decoration 4,000 €

Cost summary:

[*]Total plot purchase costs 195,600 €
[*]Deductible equity 115,000 €
[*]Financing sum building savings contract with 300 € rate for 10 years & private loan 20,000 € without interest with 100 € rate
[*]Rate for plot 400 € total

Necessary credit information & other:

Since we have to accommodate the granny flat somewhere but also want to do a lot ourselves, we estimate the costs for the house including excavation and foundation slab at 500,000 €. The entire amount would then have to be financed, as our equity with plot etc. would practically be fully committed and we want to keep a small buffer.
Big question: is this even realistic? How much more can we afford?

We have to build the granny flat because the allocation of the building plot was based on points (as almost everywhere here in the Württemberg Allgäu), and we would have had no chance without children. A cookie-cutter house would therefore be difficult (?)

Another question for you would be: where is the best place to build the ELW so that it remains financially within limits? Basement is basically out because of white tank and lifting station, making it the most expensive option. Since we are allowed to build relatively high and the neighbor also built that high, the attic would be an option without losing too much garden area. Regarding the costs for that, we are being told a range. According to the development plan, we have to build two full floors anyway.
Our goal is not to pay off the house as quickly as possible. We want to live in it for quite a while but probably not our last years and want to sell the house at the latest then.

I thank you in advance for your answers, even if I might not like them. But that way we will have a bit more clarity.

Oh, and one more note: Existing properties cost immense amounts here. E.g. built in 1970, 140 sqm, hereditary building right plot 550K, semi-detached house from 1930 with 80 sqm living space and 316 sqm plot 380K. Therefore, unfortunately, that is not the better option; we searched for about 4 years for an existing property.

Best regards and many thanks for your answers.

Is the ELW just on paper, or is it actually meant to be rented out? If rented, then the construction costs are not enough for 155m², because many things are needed twice, such as entrance, electricity/water/heating meters, kitchen, bathroom. Garden costs are significantly higher. 40m² is too much for students and too little for a couple. I don't want the ELW above me; if anything, then in the basement or an extension.

Thank you for your assessment! The ELW must be rented out for 5 years as proven. There is a loophole: in the purchase contract for the building plot it states that in the event that the construction is not carried out as agreed, a penalty of €50/sqm must be paid. So €23,700. We actually do not want to proceed that way, but theoretically it would be an option.

I do not perceive this as a so-called "loophole" and think that the municipality offered families with children this €50.-/sqm as a subsidy. You do not have children and therefore get the building plot for this additional price. I would pay this and build exactly as it suits me as a couple. The intended granny flat is not only economically a naive calculation, but it will also ruin the planning of a nice house at various points. Better leave it and be glad that you can live there on the property without outsiders—what a privilege. If the plot had been €23,000 more expensive from the start, you would have bought it just the same, so in my opinion you should think like that.

So wollen wir eigentlich nicht verfahren, es wäre aber theoretisch eine Tür.

...and why not? This way is completely legal, and people without children should not be disadvantaged either. Therefore, this is not "theoretical" but practical, indeed even from several perspectives. The previously considered way, on the other hand, is completely theoretical and would bring you a series of stumbling blocks in reality. Creating a cheap apartment with little living quality and renting it out on your own land is, in my opinion, a clear misjudgment of the consequences.

You will then get the building plot for this additional price. I would pay this and build exactly as it suits the two of us.

I think that without a separate apartment (ELW) the construction would not be approved at all. You can build differently than in the building application if you find an architect who makes two plans and doesn't get caught. If it is properly planned, a separate apartment (ELW) is worthwhile, especially if it is occupied by relatives.

I think that without a granny flat (ELW) the construction won't even be approved.

Unfortunately, I share this concern.

If planned properly, a granny flat is worth it, especially if lived in by relatives.

The question then is, what is "proper"? And what capital must be invested for that? How much more money could we invest without making ourselves unhappy?
Relatives will not be living there in the foreseeable future.