erm, a really fat peeping tom maybe? but seeeriously…... I had a similar thing (well similar in the sense it was the windowsill) with an upstairs windowsill a few years back, It cracked rather than sagged but turned out it wasnt the house settling, it was the internal structure of the sill, it had an iron (or some kind of steel) bar running through it and it had rusted and then burst the concrete sill. I actually had to replace 2 lintels too as well as one sill and some repairs to the other front upstairs sill. Depending what your sill is made from would possibly determine causes of sagginess, Here in Scotland our houses are brick built, this house was built in the 30’s and has done all the settling it’s ever gonna do, Now it’s old age that’s causing me, I mean IT to fall apart! Is your frame 2 foot wide by 6 high or the other way round? is it painted with REALLY HEAVY paint? :0) – is it possible the former tenants used to sit on it? Is there a local builder you can get to have a look-see, a pro opinion is generally the way to go (it just hurts the pocket) Direct Sunlight can definately have a detrimental effect on your building, again depending on the materials the windowsill is made from, It’s not made of cheese or something is it?