It is not very likely that you will need separate business premises - a room set aside in your home should be sufficient to keep your business records in and to use as an office. If you do this, you should be able to agree with HM Revenue & Customs (HMRC) that a certain proportion of the domestic rent/mortgage interest payments is allocated as a business expense and set against your tax bill. The allocation will be reduced if the room is used for purposes other than business.
You may also be able to allocate a proportion of cleaning materials.
Talk to your accountant, who will be able to advise you on whether or not you are able to claim a deduction for some of your household expenses, and the percentage of the total cost of your rent or mortgage that would be reasonable to treat as a business expense.
If you enter the allocation of business rent/mortgage interest in your cash flow forecast remember to adjust your domestic rent/mortgage payments when calculating how much you will have to take in Drawings to cover your living expenses.