Unless you live in a listed building or your house is in a conservation area most single storey additions, including conservatories and orangeries, to domestic houses, can be built without the need to apply for planning consent.
Your orangery or conservatory could be built under the governments permitted development rules, which allow (with some conditions and exemptions) a single storey addition of 4 metres for a detached house and 3 metres for an attached house.
These sizes are measured from the rear wall of the house as it was first built, or for an older property as it stood on 1st July 1948
The government now allows larger permitted development, doubling the size to 8 metres for detached and 6 metres for attached properties.
An application form needs to be submitted and the neighbours to the property are consulted, but the process is more straightforward than a full planning application