Ok, we are now getting into quite specific areas.
In England & Wales ONLY, the rules about permitted development rights changed on 1st October. Permitted Development rights cover what you can do without applying for formal permission - in many cases things were made easier - but bear in mind that it will have impact on the way OTHERS you are in competition with assess the ptoject (and hence have a bearing on what they will bid).
Look here if you are talking England & Wales.
http://www.planningportal.gov.uk/england/genpu b/en/1115315233153.html
Next establish quickly if this location is within an AONB or any of the other 'special' landscape categories - the planning rules are still quite tight in these areas (to stop what others see as despoiling best quality countryside).
You can look at other categories of project on this Government Planning Portal website - I have picked summerhouse out as your project. If you read the 'rules' you may find you have quite a bit of freedom.
I do not know what your comment about 30 square metres means. I think you are confusing Building Regulations and Planning Consent. BRs does not necesarily apply to structures of less than 30 square metres in floor area - conservatories are a good example. But if you are putting a decent structure up (whether less than 30 sqm or not) one wants BRs applied (insulation, ventilation, moisture resistance etc).