Built-in ovens vary in style and type, but thanks to industry standards, you can usually depend on them to be consistently sized, with only a few variables. The size of the opening may refer to base cabinet placement or to the dimensions of the countertop cutout. If you're planning on custom cabinets, the cabinetmaker or contractor will check these specs and be responsible for getting it right. If you're on your own, rely on industry standards or check the specs yourself.
Custom cabinetmakers almost always allow 30 inches in width for built-in ovens. Ovens can be 27 inches wide, but unless you're cramped for space, your oven will likely be 30 inches in width or 1/4 inch less. All base cabinets have the same depth at 24 inches, and that's not typically a concern. Depending on the oven, it might mean that the opening is regulated only by the placement of a cabinet on either side. The oven appears to be built in, but it's sandwiched between two cabinets placed 30 inches apart. If the oven is shorter than standard, it's routine to build a base to bring it up to countertop height.
Some ovens include the cooktop. This type of built-in oven is also typically 30 inches wide. The opening is cut directly into the countertop after the cabinets and countertop are installed and before laminates or tile are added. After the base cabinets are installed 30 inches apart, the countertop material -- usually chipboard or plywood -- is nailed across the top. The installer then cuts a 30-inch-wide opening directly into the countertop for the oven. Lots of oven manufacturers include a template for this, and that makes it easy. If you don't have one, take some measurements and cut the opening yourself using a circular saw. After the oven is installed, tile or laminates can then be installed on both sides to finish it.
Modular or premade cabinets use fillers to size openings. Depending on where the cabinets are set according to the dimensions of the cabinets or floor plan, your opening may be larger than you need. In this case, cut hardwood fillers for each side to make up the difference. The fillers screw directly to the sides of the cabinet face frame to finish the oven installation. It looks fine and allows more room for the oven installation, as the fillers are installed after the oven. Use the same wood species, with matching stain and lacquer. Most modular cabinets come with finished wood fillers ready to be cut to size.
Even though dimensions for cabinets and built-in ovens are standard, always check the specs first. This takes the element of surprise out of the oven installation, as some ovens require slightly more or less space, depending on the manufacturer. The specs usually indicate that a 30-inch opening can be plus or minus 1/4 inch in size. To be on the safe side, it's usually OK to build the opening up to 1/4 inch bigger than needed, as some manufacturers build their ovens exactly 30 inches wide, and fitting them into the opening then becomes difficult. Other manufacturers build ovens slightly smaller than the required 30 inches, allowing them to easily fit in a 30-inch-wide space. If in doubt, make the opening a tiny bit wider than you expect you'll need. If it's a bit too wide, nobody will notice a 1/8-inch gap on either side anyway.
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