That's a pretty tight area to be working a deep pond in. As mentioned above, make sure and check for buried lines first! A suggestion, if you haven't considered it already, would be to have a plant shelf along the edges facing the sidewalk - then the plants growing there would server as a border, and the shelf would give a safety step if (when) someone does accidentally step off the sidewalk. You also want to make absolutely sure that you go with high-quality liner material here. If you have a major leak, it could damage the house foundation over time.
One more idea that could give you a really cool effect, but would take a bit more work and planning... Where the sidewalk turns towards the house and goes up to the porch, you could remove that portion of sidewalk, dig your pond longer so it extends all the way across the front, then build a bridge over the pond to replace the sidewalk. The bridge would give your fish a nice hiding place, and by making the pond longer, you could make it more narrow and not need to come so close to the sidewalk while still maintaining a large number of gallons capacity.
One more idea that could give you a really cool effect, but would take a bit more work and planning... Where the sidewalk turns towards the house and goes up to the porch, you could remove that portion of sidewalk, dig your pond longer so it extends all the way across the front, then build a bridge over the pond to replace the sidewalk. The bridge would give your fish a nice hiding place, and by making the pond longer, you could make it more narrow and not need to come so close to the sidewalk while still maintaining a large number of gallons capacity.