I would like to have a 70cm 5 burner gas hob, but only have room for 60cm extractor hood - is this legal?

There is no problem with a 600 hood over a 700 hob. There is a problem however if wall units are allowed to overlap the hob. There must be a clear distance above the full width of the hob - usually 650 -760 depending on manufacturer. Therefore it would be best to infill at the side of the hood or if possible use a 600 chimney with a 50 gap at each side.

Sorry Tony but that is not strictly the correct answer!!
As a CORGI registered person I would first of all consult the manufacturers instructions for the hob I was fitting. They would tell me to what reg's/measurements I would have to follow!!

Chris

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"I just hope we can be free and open to each other, as we all have different ideas and will often disagree, but hopefully stay friends.

In response to the two answers that you have already received, my experience, as a kitchen designer of 10 years experience, I would advise you leave a clearance - that of the size of your hob, because - the moisture from the condensation when cooking on your hob, could be trapped in the lighting shields of the units on either side of the extractor. This will deminish the lifespan of the wall units.
Just an observation - take it or leave it - your decision.... Good luck with your project.