Extending the length of my exterior cills

You could cap the cill with a thin trim that saves you replacing anything not the window and not the cill you just cap over the existing cill that will save you time and money. A small overhang will be advisable as to get the measurements spot on is difficult for someone , ideally when you buy from Eurocell keep the receipt ,when you place on u can judge it for yourself so if not the correct size you can take back for a small cap, Eurocell are very helpful but when you collect at the depot ask if it can be returned as I think some Eurocell depot are franchised

You shouldn't cap over the exterior cill, because you will be blocking the exterior drainage holes on the bottom of the windows, that's why cills clip on to the bottom of the windows to allow the windows to drain, check your windows have been sealed properly, with sealant, hope this helps, kind regards martin mcs

Do not cap the existing cill you may block the drainage holes,replace the cill or fit a complete new frame with the correct size cill.

The correct and simple answer is no, the other companies who advised the cill would be required to be replaced are correct. Although, there are numerous issues. 1. Depending on what the cill is bedded on, cement or silicone, will determine if there is any play allowing the cill to clear the 5mm upstand at the back of the cill, to be removed keeping the window still in place 2. Its fairly standard practice for professional fitters to fix down through the cill using 100mm masonry bolts, meaning therefor the window sat on the cill would need to be removed to gain access to the fixings. 3. A further problem for simple removal is whether the frame which is sat on the cill, is fixed in place by screws threw the bottom of the window, into the cill, again therefor meaning, at the very least, the glass would need removing to access the screws. 4. The reason you DO NOT want to cap over the cill is because you will no doubt cover the drainage gap between the bottom of the frame and the cill. This is destined specifically to let any moisture drain out through the concealed drainage holes, down onto the cill and away, also to use cappit board would mean there is no correct end caps and the hole thing would look a proper mess. All in all, its probably easier, simpler and no doubt quicker to get a proper fitter to just de-glaze and remove the frame, replace the cill with the correct size and re-install, should cost approx £80-100 pending window size, job done.

Crowne Property Upgrades has said it all really. Do not cap over the cill as you will block drainage holes.

Capping over a existing cill is not ideal. However, you can cap it with facia board but making sure you are not blocking the drainage holes or you leave enough space for the water to run off.