It is unclear if this property the subject of your post is the first such property that you have 'done up' to sell on, or one in a series. The relevance is that for a trading entity (and you seem in my opinion correctly to have recognised that this is an income / trading venture, rather than being capital in nature, primarily because you bought with the intention of selling it on at a profit) trading itself is deemed to commence only when the taxpayer is in a position to fulfil orders / contracts. Until you had a house to sell and were actively marketing it, you were not trading. Therefore if this were your first such venture then the commencement rules would apply such that the expenditure would effectively "roll up" to the point that you were in a trading position - expenditure physically incurred before trading would for tax purposes be deemed to have been incurred on the first day of trade.
The rules work differently for an ongoing activity, where expenses are generally recognised as they are incurred. This is a significant over-simplification, since there would be accounting adjustments to recognise expenditure added to work in progress as a current asset, etc., (rather than to deduct expenses from income that has not yet arisen) but those expenses would have been returned in accounts for the previous period in 2017/18.
These rules are further complicated by the new "cash basis regime" where normal accounting practice is largely ignored.
Do you have an accountant or tax adviser who prepares accounts / tax returns for you?