The current rules are in the famous
Town and Country Planning (General Permitted Development) (England) Order 2015. Section 3 contains a number of special cases, none of which
AFAICS▸ is relevant here. The list of permitted developments for transport is in Schedule 2, part 8, of which this is the railways bit:
Class A – railway or light railway undertakings
Permitted development
A. Development by railway undertakers on their operational land, required in connection with the movement of traffic by rail.
Development not permitted
A.1 Development is not permitted by Class A if it consists of or includes—
(a)the construction of a railway;
(b)the construction or erection of a hotel, railway station or bridge; or
(c)the construction or erection otherwise than wholly within a railway station of—
(i)an office, residential or educational building, or a building used for an industrial process, or
(ii)a car park, shop, restaurant, garage, petrol filling station or other building or structure provided under transport legislation.
Interpretation of Class A
A.2 For the purposes of Class A, references to the construction or erection of any building or structure include references to the reconstruction or alteration of a building or structure where its design or external appearance would be materially affected.
That's regulations, so must be done under some statute, in this case the
Town and Country Planning Act 1990. That enables the orders, so does not itself permit the developments in those orders. I can't see any relevant previous statutes in the list of revocations, which is odd (but it is all long and complicated enough for all manner of nasties to be hiding in it).
Looking from the other end, most railways acquired their original rights by citing the
]Railways Clauses Consolidation Act 1845[/i]. As its title implies, that bundled together all the clauses found necessary in previous individual railway acts. To my surprise that is still on
www.legislation.gov.uk, so some part of it must be still in force. It's hard to say which parts, since the project to incorporate revisions into these statutes is working backwards and hasn't been completed this far back. In fact, I'm impressed that they claim to have listed all the revising acts with the text - not long ago few of these old acts had even been started on.
HE's development rights where they own the bridge and the ex-railway land under it are likely to be covered under roads, of course. That's part 9:
Development relating to roads
Class A – development by highways authorities
Permitted development
A. The carrying out by a highway authority—
(a)on land within the boundaries of a road, of any works required for the maintenance or improvement of the road, where such works involve development by virtue of section 55(2)(b) F177 of the Act; or
(b)on land outside but adjoining the boundary of an existing highway of works required for or incidental to the maintenance or improvement of the highway.
Class B – development by the Secretary of State or a strategic highways company under the Highways Act 1980
Permitted development
B. The carrying out by the Secretary of State or a strategic highways company of works in exercise of the functions of the Secretary of State or the company under the Highways Act 1980 F178, or works in connection with, or incidental to, the exercise of those functions.
Interpretation of Class B
B.1 For the purposes of Class B, “strategic highways company” means a company for the time being appointed under Part 1 of the Infrastructure Act 2015 F179.
Class C – tramway or road transport undertakings
...
Class D – toll road facilities
...
(See the full text for revisions information.)
Presumably HE is in one or other of those two categories.