Thanks for your question. I hope I can be of assistance.
Firstly, regarding the building. You state it is currently used as office/residential. In order to use the residential part for office this will require an application to the local planning authority for change of use. The planning department must turn around your application within 8 weeks.
Secondly, if you are carrying out works to convert the residential part to office then do check whether you need planning consent. At the very least you may need building regulations consent so do speak to building control at the council.
Thirdly, before commencing any works you should get an asbestos survey undertaken to establish whether or not there is asbestos present and how to deal with it in respect of the proposed works. This is covered by the control of asbestos at work regulations. If you sell the freehold or rent out the office then any decent solicitor will ask for this, and the building regulations consent and planning consent.
As to fire safety, there is the regulatory reform (fire safety) order 2005 which sets out requirements. You can instruct a specialist company to undertake a survey. Alternatively you could ask the fire service, they may be able to assist.
Regarding health and safety, you can check hse.gov.uk.
I would also mention the disability discrimination act, within this statute are regulations to ensure that disabled people are able to access the building and can access and use certain facilities, such as toilets.
I hope this is a useful start. If you have any further questions or which to chat this through please do not hesitate to contact me.
Thanks for accepting my answer.
If you have any further questions or queries relating to your proposed letting please do not hesitate to contact me.
Rather than refer you to the statutes themselves please check the following for guidance:
Regulatory Reform Fire Safety Order 2005:
Disability Discrimination Act 2005:
Health and Safety:
Control of Asbestos at Work:
This should provide initial guidanceas to the requirements and on-going obligations.