Do you mean that the garage is already 2 storey and they want to put another floor on top, or convert the loft space (dormers etc)? Or is the existing garage only single storey?
Is the garage attached to their property (or yours)?
Areas to object where applicable:
Out of character - taller than other properties in the area, creating semi/terrace where others are detached/semis, completely different design.
Impact on you: - overbearing/visually dominant, loss of light and/or privacy, increased noise/disturbance if very close.
general impact: loss of existing parking, lack of parking for new property.
Are there any 'special' factors like Conservation Area, impact on trees etc?
If you go to your Council's website and look under the planning policy section, you should be able to find a document called something like the current Local Plan. That will contain lots of policies which the COuncil will use to assess the application. If you can find any conflicts with those, include them in your objection.
Speak to your local COuncillor.
If you can give any further details I may be able to advise a bit more.