The Health and Safety Executive said a young boy "was hospitalised as a result".
The company had installed a conservatory at a home in Maidstone, but built the annex enclosing the external flue of a gas boiler.
The HSE said this had affected the operation of the boiler and caused a build up of carbon monoxide within the property.
HSE inspector Lisa Skinner said: "We are pleased that justice has been done today. A 12-year-old boy was hospitalised as a result of this incident. If his mother had not returned home later than usual that day she too may have been exposed to the build up of carbon monoxide and this could have resulted in a double fatality."
The firm pleaded guilty to a breach of the Gas Safety Installation and Use Regulations at the Maidstone Magistrate's Court.
Supaglazing was fined £4,500 and ordered to pay £4,500 in costs as well as a £15 victim surcharge.