It is my understanding that the housing allowance can be adjusted by the church for future needs (like a furnance going bad) but you can not adjust past payments.... The baasic rule on housing (as well as all benefits) is if you could deduct it on your (pastor) taxes then the church can give it as a benefit. See IRS pub 15 series.
Expenses of Patio by: Anonymous in NC
This expense could be considered a Housing Allowance expense item for the year as long as it does not put the total expenses for the year above the church preapproved total Housing Allowance. Before the church calendar year started the plans for building a patio could have been used as a reason of increasing the Housing Allowance for this year, but not as an afterthought.