Boundary construction garage, windows possible

Is a garage built on the boundary line, in the sense of the state building code, a building or an ancillary structure to the actual building?

I would definitely interpret it this way from Article 2: (2) Buildings are independently usable, roofed structural installations that can be entered by people. (8) 2. Garages are buildings or parts of buildings for parking motor vehicles.

... and if glass blocks "could be permissible"

They are not. My statement was still based on a ruling by the OVG NRW from 2012. Since the building regulations in NRW have now also been adjusted, the nationwide glass ban in external fire walls applies, unless your building is smaller than 50m³ gross volume (BayBo, Art. 28, Abs. 2, 1.). With a floor area of 7m*8.5m, the garage would only be allowed to be 84cm high.
If you want to read it yourself:
Higher Administrative Court of the State of Saxony-Anhalt, decision date 10.12.2015, file number 2 L 154/14
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A fixed and thus non-openable glazing that meets the fire resistance requirements of parts of a fire wall or a wall that is permissible according to § 29 Abs. 3 Satz 2 Building Code LSA as a substitute for a fire wall may become an "integrated component" of the wall and not violate the opening ban of § 29 Abs. 8 Satz 1 Building Code LSA (in conjunction with § 29 Abs. 11 Building Code LSA) (cf. OVG NW, decision of 04.04.2012 – 2 A 1221/11 –, juris, para. 10). However, the administrative court must be agreed with that the fundamental inadmissibility of glazing in the building enclosure wall according to § 29 Abs. 2 No. 1 Building Code LSA results from § 29 Abs. 9 Building Code LSA. This provision, which according to § 29 Abs. 11 Building Code LSA applies accordingly to walls that are permissible according to § 29 Abs. 3 sentences 2 and 3 Building Code LSA as substitutes for fire walls, allows fire-resistant glazing in inner fire walls if limited to the number and size required for use. This justifies the inference drawn by the lower court that fire-resistant (fixed) glazing is generally inadmissible in outer fire walls and in outer walls that are permissible according to § 29 Abs. 3 sentences 2 and 3 Building Code LSA as substitutes for fire walls
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There is also a comment by a lawyer on this (search engine "HDI" "fixed glazing"), still with the obsolete NRW exception.