Planning permission IS required for any extension which is to any elevation which faces onto a highway.
Extensions (including a porch and or a conservatory) are only permitted subject to specific criteria.
In the main part they cannot exceed 15 sqm beyond the floor area of the original dwelling house when it was first constructed. AND
a) the general conditions applicable to Section A
Section A —General Conditions as amended
1) No part of the development may be constructed within 9 metres (measured at ground level) of the route of any overhead electrical line carrying 11kv or more (High tension power cable);
2) No part of the development may be constructed within 9.1 metres of the bank of any designated watercourse under the Flood Risk Management Act 2013;
Also included from Schedule 4 of the Order
(2A) The operations specified must not be constructed (including any foundations) so as to involve the felling, lopping or limbing of any tree (other than a tree referred to in section 3(3) of the Tree Preservation Act 1993).
(2AB) The following Schedule 1 operations must not be situated closer to a tree (other than referred to in section 3(3) of the Tree Preservation Act 1993) than the required distance (a) Class 14 Extension of a dwelling house
(2B) for the purpose of paragraph (2AB) the “required distance” is
(a) for a tree with a single stem, 12 times the tree’s stem diameter; or
(b) for a tree with more than one stem, 12 times — (stem diameter 1)2 + (stem diameter 2)2 … + (stem diameter 5)2, where the stem diameter is measured 1.5 metres above ground level.
b) the floorspace (measured externally) of the extension must not exceed 15sqm beyond that of the original dwellinghouse;
c) the extension must not exceed 4 metres in height above ground level;
d) no part of the extension may be nearer to any highway which bounds the curtilage of the dwellinghouse than
i) that part of the dwellinghouse nearest to that highway, or
ii) 20 metres,
whichever is the nearer,
e) no part of the extension may be nearer than 1 metre to the boundary of the curtilage of the dwellinghouse;
f) the external finish of any walling must match that on the major part of the dwellinghouse; and
g) operations within this class are allowed only if –
(i) at least 2 car parking spaces remain within the curtilage of the dwelling house on completion of the operations; or
(ii) where there are fewer than 2 car parking spaces within the curtilage of the dwelling house prior to the operations taking place, there will be no net reduction in the area of car parking space on completion of the operations.
For the purpose of paragraph (g) the minimum size permitted for a car parking space is 6 metres by 3.25 metres.