A double garage conversion is the best value conversion a British house can do, for the simple reason that the expensive work happens once and the floor area is twice as large. It also gives a choice a single garage cannot: keep half as a garage and convert the rest, which solves the storage problem and any parking condition in the same move.
The economics are better and here is why
Filling the door opening, the floor build up, the electrical circuit and the building control application scale far less than the floor area does. Converting twice the space costs nowhere near twice as much, which is why a double is the one case where the cost per square metre starts to look reasonable.
Keeping half is usually the right answer
A partition down or across the space, with its own external door to the retained half, keeps the bikes, the mower and the freezer where they have always been, keeps off-street parking or storage available where a condition requires it, and cuts the conversion to the part you want. The partition carries the insulation line and the fire separation, which is a detail to get right rather than a difficulty.
Two rooms, or one large one
A full double conversion can be one generous room or two ordinary ones. Two rooms need two heat sources, two lighting circuits and a corridor or an arrangement where one passes through the other, which is rarely satisfactory. One large room with a defined zone at each end usually lives better.
Watch the structure in the middle
Some double garages have a central pier, a post or a beam supporting the roof over the two door openings. Removing it is structural work with a calculation and a beam of its own, and leaving it in decides the layout. Find out which you have before drawing anything.
Questions people ask about double garage conversion
Is a double garage conversion twice the price of a single?
No, and that is its main advantage. The once-only work is the larger share of a single conversion, and on a double it is spread across twice the floor.
Can I convert half now and half later?
You can, and it is worth designing both stages at once so the partition, the services and the insulation line are in the right place. Doing the second half later without that planning means opening up work you have already paid for.
Does keeping half a garage satisfy a parking condition?
Often it does not, because most conditions require space for a car rather than for storage. Read the condition's actual wording rather than assuming, because they differ and some require a specific dimension.