New building with granny flat: Photovoltaic - Electricity - Heating

Hello,

I know that some of my questions have already been touched on in other threads but were never answered in detail. So here is my specific case:

My wife and I want to build a single-family house on a slab foundation with an additional granny flat for the mother-in-law on the ground floor. We already have the plot (750 sqm).

Of course, a photovoltaic system should be installed on the gable roof with as many kWp as possible on the south side. I have already read here that the system may only be connected to one electrical circuit. So the granny flat would be permanently connected to the external grid. That much is clear.

The question that arises for me mainly concerns the heating (air-water): Can the heat pump and the rest of the heating be operated entirely from the electricity of the main apartment (with photovoltaic)? Or does that also have to be somehow divided?

We would of course like to use as much photovoltaic electricity as possible. If we were to rent the granny flat externally at some point, we would simply agree on a flat-rate warm rent. Would that comply with the law? Or does the electricity for heating costs also have to be itemized separately?

One last thing: I have seen that prefab house providers always charge significant surcharges for photovoltaic systems compared to the open market, and often you cannot choose the components as you wish. On the other hand, it is said that a system planned directly by the construction company together with the house is better integrated into the whole project. Would you still award the photovoltaic system externally? What should be considered during the house planning in this case so that the installation and integration work as smoothly as possible?

Thanks in advance!

I have already read here that the system may only be connected to one circuit. So the granny flat would be permanently connected to the external grid. That much is clear.

No, you can also play with the topic of "tenant electricity." Whether you enjoy that is another question.

The question that arises for me mainly concerns the heating (air-water): Can the heat pump and the rest of the heating be completely operated from the electricity of the main apartment (with photovoltaics)? Or does that also have to be somehow divided?

Where is the heat pump connected? If to the circuit where the photovoltaics are connected, then it's hardly avoidable.

If we were to rent out the granny flat externally sometime, we would simply agree on a flat-rate warm rent. Would that comply with the law?

No. A flat-rate rent including heating is not legal. You can still do it if the tenant doesn't complain. grin

One more thing at the end: I have seen that prefabricated house suppliers always charge hefty surcharges for photovoltaic systems compared to the free market and often you can't choose the components as you like. On the other hand, it is said that a system planned by the construction company directly with the house is better integrated into the whole project.

Nonsense. The house sellers just want to make a fat profit. They slap the same system without further planning on all 32 house types and equipment variants as the default.

Would you still outsource the photovoltaic system? What do you have to consider in this case during house planning so that the installation and integration work as smoothly as possible?

Not "still," but definitely. Consider? Either integrate it directly into the construction planning (coordinate cable runs, electrics) or simply prepare empty conduits.

Wow... that was quick. Thank you very much for the response.

I am just wondering how I should calculate the heating costs of the rented granny flat proportionally when the energy for the WP/heating comes from my power circuit, which is partly supplied by the photovoltaic system and the external grid? And even if you somehow calculate a reasonable amount for the total heating costs: how should one determine the share of the underfloor heating for the granny flat in the total pie? Is it possible to separately read the different water circuits and their respective energy demand?

Simply install a heat meter for each apartment. This way, the exact consumption for each unit can be determined, just like with an electricity meter. To be able to use photovoltaic electricity for heating as well, there is a special measurement concept (MK8). This means your meter cabinet would need space for at least 3 meters. However, I unfortunately cannot tell you how this would be handled for tax purposes.

Hello,

maybe a tax advisor can help here. I think the biggest question is how to represent this sensibly in a tax return.

We currently also have a rental agreement in a secondary apartment where we do not know exactly how much water and heating costs we consume. The rent is below average and we are only interested in the monthly rate. We have even agreed with the tenant that they can set the additional costs extra high, which gives them tax advantages. We neither receive any additional payment nor a refund for this. For electricity, we have a separate meter.

However, how he does this in his annual tax return, I do not know.

I am just wondering how I should calculate the heating costs of the rented granny flat proportionally when the energy for the heat pump/heating comes from my electrical circuit, which is partly fed by the photovoltaic system and the external grid?

That will indeed be exciting. A topic for the tax advisor, as it also concerns tenant electricity.

How should one determine the share of the underfloor heating for the granny flat in the total pie?

Separate circuits, heat quantity meters – just like in any normal multi-family house. Or you do it pragmatically. Simply install a small heat pump for each apartment. That’s how I do it in my place.

No. A flat rate rent including heating is not legally compliant. You can do it anyway if the tenant does not complain. grin

I answered too quickly yesterday. It is a granny flat and not a multi-family house. You are allowed to take inclusive rents there.