Can I fit oven under stairs?

The building regulations state of Cooker circuits in household or similar premises.. 4.4 "Precautions need to be taken to prevent the heat generated by a cooker, including the hob. creating a risk of fire, or of harmful thermal effects, to adjacent equipment or materials" Suitable precautions could include protecting the underside of the stairs with cement board (sometimes called tile board) and ensuring the built enclosure has adequate ventilation.

It would mean your cupboard becomes a kitchen and subject to all the relevant requirements. It is very doable, might need abit of paperwork, extra ventilation and maybe some fireproofing but not much.

It wouldn't contravene electrical regs but you might have already contravened fire regs. Is the kitchen the means of escape in a fire or does the staircase lead to a seperate passegeway? Look up part B building regulations. Especially important if you are letting. Its not the regs anyway that you need to worry about so much if its your own home. Are you worried about the spread of fire? A doorway between the kitchen and stairwell is useful as well as heat and smoke detectors.