Terri,
It's real hard to give you a definative answer on this. The improvement which will raise your value the most is dependant on the property and the specific market the property is in.
Is this a newer home, or an older home? If it's older, what condition is it in? Has it been updated? Look around at similar properties in the area which have sold recently or are currently listed. If most of them have updated kitchens with stone counters and stainless steel appliances and yours was last remodeled in the 70's, then you may want to remodel the kitchen. Talk to local realtors and find out what features the buyers in that area are looking for.
It really depends on the local market. For instance: here in my area north of Boston, spending 30-50k on an inground pool will not increase the value by the same amount. It's seasonal use is what we appraisers call a functional obsolescence. I'm guessing down south in say Florida, the same ingorund pool may be worth the investment because it can be used year round.
Adding square footage may help (as someone else pointed out), but be careful. Another example: In the town I live in, a newer colonial (under 10 yrs old) with 2200-2500 sqft generally sell in the mid 500's. Newer colonials with 3000-3500 sqft sell in the low to mid 600's. In this case if you had the smaller colonial and could finish a walk up attic, or put on an addition, you could bump yourself up to next price level. If you had the larger colonial and you added the same 500 sqft, it probably wouldn't help the value because there isn't a demand in this market for a 4000 sqft home.
This question needs some specific market analysis which is hard to do without detailed info on the property in question. This probably wasn't the answer you were looking for, but its the best I could do.
