LINQ method to create objects of different types
Considering that most of the options of MutateAddressData seem to be identical for both cases, I'd extract that into a method. But then you'd need yearsDB to contain items of the type TaxReturn.

So you'd basically end up with something like this:

var yearsDB = this.uow.TaxReturns.Where(x =&gt; taxReturnIds.Contains(x.Id)).ToList();

var ibAndVpb = yearsDB.Where(x =&gt; (TaxReturnType)x.TaxReturnType.Id == TaxReturnType.IB ||
                                  (TaxReturnType)x.TaxReturnType.Id == TaxReturnType.VpB);
if (ibAndVpb.Any())
{
    addresses.Add(CreateMutateAddressData( customer,  ibAndVpb));
}

var averaging = yearsDB.Except(ibAndVpb);
if (averaging.Any())
{
    addresses.Add(CreateMutateAddressData( customer,  averaging));
}

return addresses.ToArray();


This is the method that's called:

private MutateAddressData CreateMutateAddressData(Customer customer, List&lt;TaxReturn&gt; taxReturns)
{
    return new MutateAddressData
                    {
                    AddressType = AddressType.TaxReturn,
                    CityId = customer.PaCity.Id,
                    CountryId = customer.PaCity.Country.Id,
                    HouseNumber = customer.PaHouseNumber,
                    HouseNumberPostfix = customer.PaHouseNumberPostfix,
                    Street = customer.PaStreet,
                    ZipCode = customer.PaZipCode,
                    Years = string.Join(", ", taxReturns.Select(t =&gt; t.Year.ToString())),
                    TaxReturnType = (TaxReturnType)taxReturns.First().TaxReturnType.Id,
                    };
}