Are there any regulations/guidance regarding building houses over underground distribution or transmission lines?

Buried lines generate very low EMF compared to overhead.  If you post a photo of the overheads I and others on here  can probably take a stab at the voltage. That will tell you the likely ranges of types and depth of buried cables.

It will depend rather on exactly what is there and what is proposed.  The sort of lines that go on wooden poles with a cross bar become a plastic clad cable of a few inches in diameter and no more than a metre or two down. The high voltage 6 arm metal pylons become something considerably more chunky ;-)

The lines can only be routed in  way that they are not exactly built directly over - they may be under road, gardens, paths and driveways, but not under any heavy masonry or multistory structures without bridging lintels etc, usually avoided.
The properties in the direct  path will have deeds clauses that require access to the affected parts to be maintained, so network operator permission is needed  to dig deeper than some depth,  or to build anything more permanent than a wooden shed or fence panels that can be lifted away or demolished  in a hurry  if there is ever an emergency. (think road digging crew and digger or forklift arriving at 1AM and lots of orange flashing lights kind of hurry, same as they do for gas leaks and similar ).
It is most unlikely to ever be required, but the access restrictions are needed.
Those properties on the route may end up with odd shaped boundaries and or lower prices, in some cases there may be annual 'easement' payments from the operators to the land owner  - much as there is if there is a telephone pole in your front garden and your solicitor was on the ball.

It will vary but it can be done. Sometimes it is easier to take a new cable round the outside edge of the development.

Mike.

Hmm There will be a label on those pylons showing the voltage but lets just assume 275kV for now. Having just spent  ten mins or so translating Perygle Marwolyth  to 'peril of expiry' I'm not keen to continue..   I do not think it will tell me the voltage or grid or supergrid.

Such things go underground as one of three forms

1)   Trough: the separate cores of the cable lie in a concrete trough, maybe as shallow as  0.3 m  below ground, but also only 0.15 m apart.
At that voltage unfilled trench is rare, and likely to be deeper.

This is not for long runs or building over, but needed where feeders come to the surface to go into buildings, or perhaps running beside a railway, road or river or to hop over a bridge or up to surface into a transformer or switching station.

2)  Direct buried: the separate cores of the cable are laid directly in the ground, for 11 and 33kV lines  typically 1 m below ground and 0.3-0.5 m apart, for your sort of HV, deeper and fatter cables, and with concrete backfill.

https://www.cablejoints.co.uk/upload/400kV-Undeground-Cable-Construction---Installation-Trenching-and-Jointing---National-Grid-UK.pdf

Most likely under a road for the sort of situation you describe.

3) Tunnel: the cable is carried in a tunnel typically up to 20 m below the surface

Expensive but needed for example under London This can be built over.

This website has some pics illustrating  the sort of thing, but the cable sizes are more for the 11kV and 33kV cables that supply local substations.

https://www.emfs.info/sources/underground/types/

Mike.

Also be aware that "building over" the cables MIGHT mean that they will be under the new housing estate, but not under the actual houses. Under the new roads for example.