I can think of two ways. First, the increase in floor space or volume. The extra floor space divided by the original floor space is the proportional increase of floor space. If the height of the extra space is uniform and the same as the original height, the volume increase is the same as the floor space (area) increase, but if the shape of the extra space is different then the increase of volume is disproportional and specific measurements would be required to determine the extra volume and therefore the proportional increase in volume.
Second, the cost of furnishing plus the cost of decorating the addition. You would need to have a value on the existing furnishings and decor before you could determine the increased cost (or value) as a proportion.
(The value of your whole property would increase in proportion to the value of the addition, and you could find out what this was by having the property valued immediately before and after the addition. This would give you a proportion by subtracting the original valuation from the new one and dividing this difference by the original. That would be the proportional increase. Your bedroom has a proportional value compared to your property value, so if you were just considering proportional increase of your bedroom only you would need a formula. Let V be a value associated with the whole property and v be a value associated with your bedroom before any additions then v/V is the proportional value of your bedroom to the value of the whole property. v/V could also be the proportion of your bedroom's floor space or volume to the floor space or volume of the whole property. When the whole property is revalued at V1, and the only additions were in your bedroom, then you could argue that (V1-V)/v represents the proportional increase of the value of your bedroom.)
I hope this helps.
