Do I need Building Regulation Approval to construct a porch?

No - Provided the floor area is 30 m2 or less, the glass within the doors and windows is toughened or laminated safety glass, there is an external quality door between the porch and the rest of the dwelling, and it has not been built over a Public Sewer

You can establish the position of any Public Sewers that are within or adjacent to your property, by contacting Northumbrian Water Ltd.