The answer depends upon what the lease says and the nature of the works you want to carry out. Leases normally control the alterations that a tenant can carry out. Generally the shorter the lease the less the tenant will be able to do.
Various types of alterations are usually dealt with in the lease as follows:
Structural alterations – the tenant will not normally be able to undertake these
External alterations – the tenant will not normally be able to undertake these
Non-structural internal alterations – the tenant will be permitted to carry these out subject to getting the landlord’s consent (which it cannot unreasonably withhold or delay)
Non-structural partitioning – the tenant should be able to erect and remove non-structural partitioning without the landlord’s consent (but may need to notify the landlord of what has been done).