You do not have to give permission for ‘improvements’ to the property. It is your decision whether these take place or not. If the kitchen is being renovated due to issues that need repair work then it is likely that you will have to give your landlord access when it is reasonable. If this is the case they will need to inform you of the work, give you notice and provide you with dates of work and details of any contractors.
Landlords are responsible for the structural upkeep of the property which includes making sure that all walls and plaster are safe and fit for purpose. If you find the walls require plastering, notify your landlord as soon as possible.