A.Rob Corbo, a general contractor in Elizabeth, N.J., responds: After you've double-checked the dimensions of your window to verify that it meets egress window code, I'd recommend that you also double-check the conditions around the proposed opening in the foundation.
If excavation will be required to accommodate the new window, be aware that making grade changes could introduce complications. You don't want water running toward the house, so you may need to add money to the budget to address that issue. You'll also need to temporarily support loads over the opening with a 4-by-8-inch header that's long enough to hold up the joists above the window opening plus two extra joists on each side. Install the temporary header close to the wall, but leave enough room — 2 or 3 feet should be enough — to work.
While you can build the permanent header out of concrete-filled lintel blocks supported by angle irons, an easier option is to use a precast reinforced-concrete lintel. These are available at most building yards, and will need to be sized to your specific opening and cut to length on site. Because these lintels may develop cracks over time, I like to reinforce them with angle iron as added insurance against callbacks.
When cutting an opening in an existing block wall, try to align it with existing mortar joints, and size the new lintel so that it is supported by at least 6 inches of block on either side. Though structurally unnecessary, a pair of angle irons provides extra support that helps eliminate minor cracking in doubled-up precast lintels (Section A).
The first course of block below the mudsill will house the lintel, which should extend roughly 6 inches in either direction past the window's rough opening. Using a level, mark the lintel and window roughs on the foundation wall, aligned if possible with existing mortar joints. The outline will resemble a rather fat T. Using a masonry drill held as level as possible, drill through the wall at all six corners to mark your roughs on the exterior wall of the foundation. Use a circular saw or a grinder equipped with a masonry or diamond blade to score the wall on the interior and exterior markings. This will create plenty of dust, so be sure to protect yourself and the room.
Once the wall has been scored, start to chip away, starting in the center of the opening and working toward the perimeter. We use a rotary hammer but always have a 3-pound lump hammer and cold chisel close at hand; we periodically remove the debris underfoot to keep the work area clean.
Once you've opened up the wall, you'll have a number of partial concrete blocks that will need to be filled or replaced. Repair the opening as required, adding wire mesh to the cavities to give your mortar something to hold on to. Finally, install the lintel, shimming or jacking it flush to the sill and packing the bearing shelves you've created underneath with mortar.
Once the lintel is secure and the mortar has set, you can go ahead and remove the temporary header and posts, apply stucco as required, and install the window.