Changing tenancy ageeement from single to joint

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My partner has approached his letting agency to ask if he could change his tenancy agreement to joint so I can move in and they have said they are only willing to do this if we pay £150 fee for changing the tenancy and a whole
New deposit (around £550) and they will refund the initial deposit he paid afterwards.

Is this legal? I am struggling to understand why the change of tenancy fee is so high.

Also worth nothing I am in Scotland.

I thought Scotland had banned charging fees to tenants.

Is that a requirement ? I remember moving in with an Ex a few years ago and just moving in he never told his ketring agency until we moved out and they saw me and they didn't care.
Is that even legal surely its just a way to extort money

I spoke with shelter Scotland and they do not think this is legal in terms of the fee, but thought the 2nd deposit may be more to do with the deposit scheme.

We have received another email this morning stating that my partner signed a declaration agreeing he would pay £150 if he wished to change the tenancy policy.