Yes, and it is the part of a conservatory most likely to be skimped, because nobody can see it once the floor is down. A conservatory is a building sitting on the ground and it settles, cracks and pulls away from the house if the base is wrong, which is not something a better frame or a better roof can rescue later.
Conservatory footings: what the base consists of
A trench fill or strip foundation under the dwarf wall taken down to stable ground, the dwarf wall itself built off it, a floor build up of hardcore, sand blinding, a damp proof membrane lapped into the wall's damp proof course, insulation and a concrete slab or beam and block floor. The damp proof membrane and its junction with the wall matter as much as the concrete: that is what stops ground moisture arriving in the finished floor.
What decides the foundation depth for conservatory bases
Soil type first. Shrinkable clay moves with moisture and needs deeper foundations than gravel. Trees second, and this is the one people underestimate: a mature tree near the footprint can require a substantially deeper foundation because of the moisture it draws from clay, and the species and distance both matter. Then the existing house foundations, because the new base should not undermine them, and any drain or service in the way. A depth quoted before anyone has dug a trial hole is an assumption.
Drains, sewers and the thing found on day two
An inspection chamber or a drain run in the footprint is common and solvable, and it must be dealt with properly rather than built over casually. Building over or close to a public sewer needs the water authority's agreement, and a private drain may need diverting or a new chamber with an accessible cover. This is the single commonest cause of a price changing after work starts, which is why it belongs in the quote as a stated assumption with a stated rate if it is wrong.
Does a conservatory need foundations if it is exempt? Exempt does not mean unregulated
A conservatory may be exempt from the building regulations and it still has to stand up, stay dry and not damage the house it is attached to. The structural and moisture requirements exist as good practice whether or not an inspector visits, and the absence of an inspection is precisely why the base is where corners get cut. Asking for the assumed foundation depth in writing is the cheapest protection available.
How long do conservatories last? What a good base buys you
A conservatory with a sound base, a good frame and a properly detailed junction with the house is a long lived structure, and the roof is usually the first thing to need replacing. One built on an inadequate base shows it within a few years as cracking at the junction with the house, doors that stop closing and a gap opening at the flashing. Those are expensive to correct and impossible to correct cheaply, which is the whole argument for spending on the part nobody sees.
Other questions people ask
How deep do foundations for a conservatory need to be?
It depends on the soil, on nearby trees and on the existing house foundations, so it is determined on site rather than from a table. Shrinkable clay with a mature tree nearby is the combination that drives depth hardest.
Can a conservatory be built on an existing patio?
A patio slab is not a foundation. It may be possible to build off an existing base if it was constructed for the purpose and can be verified, and a paved terrace laid on sand is not that.
What if there is a drain where the conservatory goes?
It is common and solvable. A public sewer needs the water authority's agreement, and a private drain may need diverting or an accessible chamber. It should be identified at survey and priced, not discovered.
How long does a conservatory last?
With a sound base and a good frame, a long time, with the roof usually the first element needing replacement. A poor base shows within a few years as cracking and doors that stop closing, and it is the hardest fault to put right.