An extension must function well, and it should look as if it has always been there.

Bricks

Although you should be able to match the colour of bricks fairly closely, modern bricks may not be the same size as old bricks. You may be able to find suitable secondhand bricks, but cost could rule this out. If you are working with different sized bricks, avoid butting directly up to the existing brickwork - even a slight recess will minimize the difference.

Windows and Doors

If the existing style is not available ready-made, have matching window frames and doors (both external and internal) made by a local joinery.

Roof Pitch

The pitch should match or be sympathetic to the existing slopes and angles of the roofline. A flat-roofed extension on a pitched roof house always looks like an afterthought. If the location or your finances don't allow for a proper pitched roof, raising the roof at an angle for four or five rows of tiles will soften the effect. Alternatively, consider creating a roof terrace (take that decision early as it will affect construction).

Roofing Material

Slates and tiles can look conspicuously new for years. To achieve an instant weathered look, buy secondhand. If this is not feasible, take old tiles from the least visible part of your roof and use those on the new extension. Put the new tiles in the less noticeable position.