declare @tmp table(
       Postalcode nvarchar(10),
       Street nvarchar(100),
       Housenumber int,
       IsSeparator bit
       );

insert into @tmp values
 ( 12345, 'SomeStreetName',  188    , '0' )
,( 12345, 'SomeStreetName',  212    , '0' )
,( 12345, 'SomeStreetName',  214    , '0' )
,( 12345, 'SomeStreetName',  214    , '1' )
,( 12345, 'SomeStreetName',  236    , '0' )
,( 12345, 'SomeStreetName',  238    , '0' )
,( 12345, 'SomeStreetName',  238    , '1' )
,( 12345, 'SomeStreetName',  242    , '0' )
,( 12345, 'SomeStreetName',  244    , '0' )
,( 12345, 'SomeStreetName',  246    , '0' )

select *,
	GroupNumber = '???'
from @tmp t
order by t.Housenumber

select *, GroupNr = 1 from @tmp where Housenumber <=214
union
select *, GroupNr = 2 from @tmp where Housenumber > 214 and HouseNumber <= 238
union
select *, GroupNr = 3 from @tmp where Housenumber > 238
order by HouseNumber

select *
 ,GroupNumber = (
  case 
   when HouseNumber <= 214
    then 1
   when HouseNumber <= 238
    then 2
   else 3
   end
  )
from @tmp t
order by t.Housenumber

declare @tmp table(
       Postalcode nvarchar(10),
       Street nvarchar(100),
       Housenumber int,
       IsSeparator bit
       );

insert into @tmp values 
 ( 12345, 'SomeStreetName',  188    , '0' )
,( 12345, 'SomeStreetName',  212    , '0' )
,( 12345, 'SomeStreetName',  214    , '0' )
,( 12345, 'SomeStreetName',  214    , '1' )
,( 12345, 'SomeStreetName',  236    , '0' )
,( 12345, 'SomeStreetName',  238    , '0' )
,( 12345, 'SomeStreetName',  238    , '1' )
,( 12345, 'SomeStreetName',  242    , '0' )
,( 12345, 'SomeStreetName',  244    , '0' )
,( 12345, 'SomeStreetName',  246    , '0' ) 
,( 12346, 'SomeStreetName',  88    , '0' )
,( 12346, 'SomeStreetName',  112    , '0' )
,( 12346, 'SomeStreetName',  114    , '0' )
,( 12346, 'SomeStreetName',  114    , '1' )
,( 12346, 'SomeStreetName',  136    , '0' )
,( 12347, 'SomeStreetName',  1    , '0' )
,( 12347, 'SomeStreetName',  3    , '0' )


SELECT   t.*
        ,DENSE_RANK() OVER (PARTITION BY t.Postalcode
                            ORDER BY ISNULL(b.Band, 9999999)-- you can use max int here
                           ) as GroupId                                        
FROM        @tmp t
CROSS APPLY(SELECT MIN(Housenumber) Band 
            FROM  @tmp b 
            WHERE b.Postalcode = t.Postalcode
              AND b.Housenumber >= t.Housenumber
              AND b.IsSeparator = 1) b
 WHERE t.IsSeparator = 0