Your rights to carry out any alterations to the premises once you are the tenant will be set out in the draft lease.
The lease may say you cannot carry out any alterations to the premises. You should never agree to such a total restriction.
Alternatively, the lease may say you cannot carry out any alterations to the premises unless you first get the landlords’ consent. This is better from your point of view, but not because it offers the possibility of landlords’ consent – the landlords might simply refuse consent. That would make it no better in the end than the absolute prohibition just described. The reason it is slightly better than the first alternative is that, although the landlords can arbitrarily refuse their consent in many situations, they are not allowed to unreasonably refuse consent to improvements. This is a protection given by the Landlord and Tenant Act 1927 whenever alterations are subject to landlord’s consent.
Whether an alteration is an improvement is judged from your point of view. If an alteration would allow you to make better use of your premises, then it’s an improvement and the landlords cannot withhold their consent unreasonably.
The most lenient option you are likely to be offered is that there is an absolute prohibition against some works (probably structural works), while others are allowed if you obtain the landlord’s consent first (which cannot be unreasonably withheld for alterations of all kinds, not only improvements). Finally, purely minor or internal works can be carried out without needing to obtain permission at all. Office tenants often request it to be specified in the lease that they can erect partitioning without needing consent.
That mixture is the one you want to go for. Before you sign the lease, you should give careful consideration to what alterations you might need to make in the future and then check that the lease will allow you to do them when the time comes.
If you get approval from the landlords for works you want to carry out, that’s only part of the story – you should also consider what your obligation is going to be at the end of the lease. Some leases require the tenants to remove all their alterations and, in fact, to restore the premises to the exact condition they were in when the lease was signed. You can imagine how onerous that might turn out to be for anyone carrying out ambitious and far-reaching changes. If you are going to do a lot of changes, you should seek to ensure the lease you are going to sign doesn’t require you to reinstate the premises again. The landlord might be willing to agree to that if the changes you are proposing are a positive improvement to the premises, which will make the premises more lettable to a new tenant. But if what you’re doing is only suitable for your own business, it won’t be unreasonable for the landlord to want those changes undone.
As well as getting your landlord’s consent for significant alterations you plan to make to the premises, you need to remember that you must also comply with statutory requirements. The Construction (Design and Management) Regulations 2007 contain many regulations you need to comply with. It’s also a requirement that you keep a full and detailed record, and you may be under an obligation to produce that to the landlord.