Ordered window with wrong sill - how to fit deeper one?

Cill profiles should be available from your local timber merchant (but probably not a shed)

Malcolm

The sill doesn't need to be a very sophisticated piece of joinery, but it does need to be jointed so that there isn't a ready path for moisture to penetrate.

I would be thinking about slicing off the old sill from the rest of the bottom rail of the window frame - by slightly (say 10mm) undercutting it relative to the rest of the frame - but otherwise not disturbing the bottom rail of the frame. Either saw it off near flat with a circular saw, then create a rebate with a router - or possibly do the whole job with the router. Beware of buried metal fixings.

However *first* make up your new sill. You may be lucky a find a timber merchant that can supply a suitable donor-profile to work from

- otherwise making it up from scratch isn't too bad. Think about rain run-off paths, rounded edges and a drip-groove on the underside.

Join the new piece with both glue (polyurethane or epoxy) and fixings (capped-over screws, biscuits, dowels) - and cramp up the joint really well.

Don't know what you have int the way of power tools (and straight guides for them) - this *could* be done handtools only - but you'd have to be confident in planing up a very straight flat edge over the whole length where you're gluing the new sill onto the base of the window.