You don't need biogas, your house primarily needs _more_ insulation and probably a controlled residential ventilation system with heat recovery. An air-water heat pump + photovoltaics could be an alternative, your energy expert should calculate it.
More insulation can also be a substitute measure for the renewable share.
An air-water heat pump would solve all problems, even without photovoltaics. But this is not desired.
Above all, look where it is defined that this is sufficient according to the Energy Saving Ordinance.
That is not stated in the Energy Saving Ordinance but in the Renewable Energies Heat Act. Thanks to acute boredom, I have looked it up for you.
§ 3 Obligation to Use
(1) Owners of buildings according to § 4, which are newly constructed, must cover the heating and cooling energy demand by the proportional use of renewable energies in accordance with §§ 5 and 6. Sentence 1 also applies to public authorities if they newly construct public buildings according to § 4 abroad.
(2) Public authorities must cover the heating and cooling energy demand of already constructed public buildings according to § 4, which they own and are fundamentally renovated, by the proportional use of renewable energies in accordance with §§ 5a and 6 paragraph 2. Sentence 1 also applies to public authorities if they fundamentally renovate public buildings according to § 4 abroad.
(3) Public authorities must ensure that already constructed public buildings according to § 4, which are in their possession but not ownership, assume a model function during fundamental renovation that meets the requirements according to paragraph 2. In the case of rental or lease of buildings, this is ensured if
1.
primarily buildings are rented or leased that already meet the requirements according to paragraph 2,
2.
secondarily buildings are rented or leased whose owners commit to meeting the requirements according to paragraph 2 in case of a fundamental renovation.
Sentence 2 does not apply if buildings are rented or leased by public authorities only temporarily.
(4) The federal states may
1.
adopt their own regulations for already constructed public buildings, except federal public buildings, to fulfill the model function according to § 1a and deviate from the provisions of this law for this purpose, and
2.
establish an obligation to use renewable energies for already constructed buildings that are not public buildings.
§ 5 Share of Renewable Energies in New Buildings
(1) When using solar radiation energy according to number I of the annex to this law, the obligation under § 3 paragraph 1 is fulfilled if at least 15 percent of the heating and cooling energy demand is covered thereby.
(2) When using gaseous biomass according to number II.1 of the annex to this law, the obligation under § 3 paragraph 1 is fulfilled if at least 30 percent of the heating and cooling energy demand is covered thereby.
(3) When using
1.
liquid biomass according to number II.2 of the annex to this law and
2.
solid biomass according to number II.3 of the annex to this law
the obligation under § 3 paragraph 1 is fulfilled if at least 50 percent of the heating and cooling energy demand is covered thereby.
(4) When using geothermal energy and environmental heat according to number III of the annex to this law, the obligation under § 3 paragraph 1 is fulfilled if at least 50 percent of the heating and cooling energy demand is covered by the installations for the use of these energies.
(5) When using cooling from renewable energies according to number IV of the annex to this law, the obligation under § 3 paragraph 1 is fulfilled if the heating and cooling energy demand is covered at least to the extent of the share according to sentence 2. The decisive share is the share according to paragraphs 1 to 4 for the renewable energy from which the cooling is generated. If the cooling is generated by a thermal cooling generation system by direct supply of heat, the share that also applies in the case of pure heat generation (without cooling generation) from the same energy carrier applies. If the cooling is provided directly by the use of geothermal energy or environmental heat, the share of 50 percent of the heating and cooling energy demand applying also to heat generation from these energy carriers applies.
Sounds bad at first, but §7 knows substitute measures...
§ 7 Substitute Measures
(1) The obligation under § 3 paragraphs 1 or 2 is considered fulfilled if the obligated parties
1.
cover at least 50 percent of the heating and cooling energy demand
a)
from plants using waste heat in accordance with number V of the annex to this law or
b)
from combined heat and power plants (CHP plants) according to number VI of the annex to this law;
§ 5 paragraph 5 sentence 3, § 6 paragraph 1 sentence 1 and § 6 paragraph 2 apply accordingly,
2.
take measures to save energy according to number VII of the annex to this law, or
3.
purchase district heating or district cooling according to number VIII of the annex to this law and cover the total heating and cooling energy demand at least to the extent of the share according to sentences 2 and 3. The decisive share is the share according to § 5, § 5a or number 1 for the energy from which the district heating or district cooling wholly or partially originates. For the calculation according to sentence 1, only the amount of district heating or district cooling used that is calculated to derive from renewable energies, from plants using waste heat, or from CHP plants is credited.
(2) The obligation under § 3 paragraph 2 is also deemed fulfilled if solar thermal systems according to number I of the annex to this law are operated on the roof of the public building by the owner or a third party when the heat or cooling generated by these systems is made available to third parties to cover the heating and cooling energy demand of buildings and is not used by these third parties to fulfill an obligation under § 3 paragraphs 1 to 4.
And finally number V of the annex
V.
Waste Heat
1.
If waste heat is used by heat pumps, numbers III.1 and III.2 apply accordingly.
2.
If waste heat is used by air-conditioning systems with heat recovery, this use is only considered a substitute measure according to § 7 paragraph 1 number 1 letter a if
a)
the heat recovery rate of the system is at least 70 percent, and
b)
the coefficient of performance, which is determined from the ratio of the heat recovered and used from heat recovery to the electricity consumption for operating the air-conditioning system, is at least 10,
3.
If cooling that is technically utilizable and supplied directly with waste heat by systems is used, number IV.1 applies correspondingly with the exception of sentence 1 letter a.
4.
If waste heat is used by other systems, this use is only considered a substitute measure according to § 7 paragraph 1 number 1 letter a if it complies with the state of the art.
5.
Proof within the meaning of § 10 paragraph 3 is
a)
in the case of number 1, the certification by an expert and the environmental label “Euroblume,” the environmental label “Blue Angel,” the test mark “European Quality Label for Heat Pumps,” or an equivalent proof,
b)
in the case of number 2, the certification by an expert or the certification of the system manufacturer or the specialist company that installed the system,
c)
in the case of numbers 3 and 4, the certification by an expert.
Many roads lead to Rome. General contractors mostly know only "their" way. Good energy consultants know several others.