james

It's clear that the cost of the extention will add value to the property over and above the actual cost of the work, in your position it would make sense to agree to her request.

for example: Value of property without ext. £250,000
Value with extention £300,000
Cost of extention £30,000
New Value (less costs) £270,000
making an assumption of your %age at 20%

20% of £250K = £50K 20% of £270K = £54K

in this scenario you cant lose

Bill