I am trying to replace a 20 year old built under electric oven in the house I have just moved into.

was it a built under oven on rails or in an oven housing? a standard aperture for a built under oven is 600mm wide and for a built in is 560mm wide. Any bigger is no problem, any smaller will be....... modern standard carcass height is 720mm with an internal measurement of 680mm. The base will then be built up to suit the height of the oven. Finally, the depth to the wall from the front edge of the carcass should be a minimum of 560mm. So long as you have these dimensions for the hole you have left then a modern built in oven will fit. Hope this helps

I fully agree with Mr. & Mrs. above nothing much has changed in the last 20 years, a single oven will usually be in a housing and a double will be on rails in the aperture, most should fit and the doubles will come with the fitting rails.

You will be able to replace your oven if it is 600mm wide standard cabinet. you will need to check the power rating of new oven if you want a powerful oven. check with an electrician if any doubt because you may need to uprate your elec supply.Heights of built in/under ovens do vary but not in width unless it is built under on metal runners.this type is usually more powerful.