If you are planning to do a half and half conversion that retains the front part of the garage as a store room for bikes, motorbikes, electric scooters etc then the dividing door has to meet fire regulations, and capable of stopping the spread of fire and smoke for 30mins to allow for a means of escape. It also needs a fire smoke alarm to be fitted.
If you are making any visible changes to the front of your property, or fitting additional windows to the side, then you will have to apply to your council for Change of use permission. Depending how close your side door is to your neighbours boundary, that too may have to have fire safety features such as opening outwards, and having any glass panels capable of holding a fire for 30 mins, and not melting around the frame.
We have done a half conversion lengthways conversion to reduce our double Garage to a single, creating a downstairs bedroom for disabled use with its own en-suite. We ended up having to raise the garage floor level after knocking through to the main house, to prevent a step down between both rooms. Garages are often build with a slope down towards the front doors as a safety feature to drain away and provide ventilation for any potential fuel leaks from the vehicles stored within. Ours had a difference front to back of 15 cm, so now we have a step down into the remaining double garage 2. Out of our utility room.
We are pleased that we did this, and externally it just looks like it’s always been just a single garage with a side window to the conversion, Our neighbour has had her single garage done half and half remaining the original front garage door, but complains that she has to keep the light on all day whilst working in the back half, the glazed side door panel is not sufficient to provide a decent level of daylight.