The building regs prefer soakaways as a way of dealing with rainfall because they allow it to be soaked into the ground over a long(er) period thus helping to prevent some of the disasterous flooding we've seen in the last year. They also avoid sewage plants becomming overloaded during periods of peak rainfall, the outcome of which is, er, unpleasant.
The calculation of the size is non-trivial because a lot of the data you need is not readily available. In particular you need to know the likely sustained rainfall rate in your area as well as the permeability of the ground. Your local BCO might be able and willing to help if you ask.
30m2 is quite large, consider 1cm of rain falling reasonably quickly, that's 0.3m3 of water. If you are on sand then no worries but if you are on clay then you probably have to allow for storing all of that in the short term. As most normal domestic soakaways I've seen are about
1m2 that's a third of the volume without any other considerations.
In practice of course everyone just digs a big hole and fills it with rubble but you could look at some of the comercially available high-tech solutions based around a plastic latticework and a permeable membrane - sort of like upturned milk crates - basically anything which provides some temporary storage so that the water can soak in over time.
If you have a 70's house you may find the soakaway is rather too close for present day standards. Ours was about 2 metres away until we built our extension, now it's the minimum permitted 5m.
HTH Calvin