If you have a spare room, you can take advantage of the government's "Rent a Room" scheme which lets you earn tax free income up to a certain threshold from letting out furnished accommodation in your home. You can find out the threshold you can earn that will be tax-free on the Gov.uk website. Taking on a lodger can prove a tax efficient method of supplementing your income but it's important to get the details right. A professionally written Lodger agreement sets out matters such as rent, deposit and the rights and responsibilities of both lodger and landlord. It also enables you to ask your lodger to leave if you need to get your room back. For further information, read Taking in a lodger.
Sharing property ownership or renting out parts of your property to others can be a useful way of splitting costs or supplementing your income, but ensure that you protect your interests by formalising things with relevant documents.
If you decide to take on lodgers, you should make sure you have a Lodger agreement at the start and set out the various house rules in a House rules for lodgers document that the lodger can refer to easily, as well as preparing an Inventory. Create a Car parking licence to rent out a space that you're not using.
Rent out spare office space to other businesses or even rent out part of your home to your own business. Use an Office sharing agreement or a Home office rental agreement if you want to rent out office space. If someone else is helping you with the costs of buying a property, a Declaration of trust - beneficial interest (for England and Wales) sets out the important details regarding the beneficial interests of non-legal owners. Finally a Party wall agreement will help smooth the way for any building work that may affect your neighbours with whom you share boundaries.