Planning permission IS required for any extension which is in front of any elevation facing onto a highway.

Extensions (including a porch) are only permitted subject to specific criteria, in the main part they cannot be more than 15 sqm beyond the floor area of the original dwelling house when it was first constructed. There are further conditions applicable, on pages 15 of the Permitted Development Order (for the general conditions) and Class 14 on page 16 for those applicable to any extension via this link.