A garage is a relatively light structure, so your footing will only need to be about 1m max depth. They need to be deeper near trees, but it looks like that isn't an issue. Depends on the geology - clays shrink & swell, sand/gravels have higher bearing capacities but settle a little.
If you get an architect to design it for you, they may ask for a small ground investigation (GI) to determine the existing building foundations as the garage will be attached to the house and also for testing the soils. Soils generally need to be tested for a) shrinkage potential and b) chemical attack on concrete. They would need this info to design the foundations, floor slab etc and attach the garage to the house. It would also be useful to see where the services are going.
This is where I can help you as I work for a GI firm !

- sorry about the shameless advertising!.
I quote lots of clients for this type of thing. It looks like you might need to consider a day of hand dug pits or window sampling to get some soil samples for testing and also to examine the house foundations. Typically this sort of GI will cost in the region of £1500-1800 for sitework, testing & reporting. You can dig the pits and get soil sample yourselves to save money but my guess is that the designer of your garage would need a professional pit log & drawing/measurements of the foundations. Soils testing for plasticity/sulphate attack will set you back about £50/sample.
I would seek the advice of the local authority planning dept and a design/build firm (we deal with lots of those!). If you need a quote for a GI or want soils testing, PM me. We cover most of the UK.