Usually the rental business does not begin until the first property is let.
Any expenses incurred before the first let must be solely for the rental business and must not be capital expenditure.
Any qualifying pre-letting expenses are treated as incurred on the day the rental business commences, which to confirm, is usually the day the property is first let.
After the first property has been let, any later expenditure leading up to the letting of the second and later properties is part of the rental business and can be deducted - provided it is incurred wholly and exclusively for the purpose of the business and isn’t capital expenditure.
You can find more details here:
PIM2505 - Beginning and end of a rental business: commencement