Above door heaters , ideally dimplex if you have the money , iam a sparky and was no way paying £150 for a 3 kw heater so I purchased x3 premiair wall heaters 2 kw each and after 30 mins on iam sweating me bollocks off lol. They have stat display , programmable controls , 2 stage heating and best of all a fan mode for summer and also have moving vents to spread the heat around , and they are remote control as well. Around £40 each and my garage is 7.5 m x 6.9m x 4.9m pitched roof with no insulation but plasterboarded , but has a insulated roller shutter door.
Cheers paul