First of all, are you a sole trader or partnership business or do you run a limited company?
Generally speaking, any costs entailed in building or converting your toilets can be capitalised and then depreciated over the life of the asset. They should be tagged to one of the fixed asset codes and it’s also best to keep a fixed asset register separately, e.g. on a spreadsheet. These costs will sit in your balance sheet.
Once the toilets are completed, you would then start to depreciate them. Work out roughly how long you expect them to last. If this is, say, 5 years, then each year you would transfer a fifth of the cost from the balance sheet to the profit & loss account (by way of a journal which debits depreciation charge and credits accumulated depreciation in the balance sheet).
At the end of the five years you will have “written down” the asset to nil.
On your tax return, you must add back depreciation as it is a disallowable expense. However you can claim capital allowances instead - usually the full cost as part of the Annual Investment Allowance. Whether you use this or claim a general 18% writing down allowance will depend on whether you are making profits or losses.
See the HMRC website for more details on capital allowances. https://www.gov.uk/work-out-capital-allowances/work-out-what-you-can-claim