Estoppel is a dandy legal word whose definition made my head spin. My paraphrasis is "you can't have it two ways." For a dictionary of legal terms:[ur;]www.butterworths.com.au/legalwords/default.htm [/url]

Just because I know that you enjoy having your head spin, Dr Bill ~

There are several different types of estoppel. Here are the major variations under Michigan law.

Collateral estoppel - Generally, collateral estoppel precludes relitigation of an issue if a question of fact essential to the judgment was actually litigated and determined by a valid and final judgment, the same parties had a full opportunity to litigate the issue, and there is mutuality of estoppel.

Equitable estoppel - Equitable estoppel can arise when: (1) a party, by representations, admissions, or silence, intentionally or negligently induces another party to believe facts; (2) the other party justifiably relies and acts on that belief; and (3) the other party would be prejudiced if the first party were allowed to deny the existence of those facts.

Promissory estoppel - To establish a claim based on promissory estoppel, there must be: (1) a promise; (2) that the promisor should reasonably have expected to induce action of a definite and substantial character on the part of the promisee; (3) which in fact produced reliance or forbearance of that nature; and (4) in circumstances such that the promise must be enforced if injustice is to be avoided.

You can read about cases involving estoppel here:

http://courtofappeals.mijud.net/Digest/topiclists/Estoppel_pg1.htm

laches - long ago I heard short definition: "sleeping on your rights". In case mentioned, I think city could have had defense that it did not learn of the fence until six months after it was built. Also, it seems llikely that a permit would have been required, so if fence were built without permit, laches would not be a defense. Too bad Keiva isn't with us.

The application of the doctrine of laches requires the passage of time combined with a change in condition which would make it inequitable to enforce the claim against the defendant. The defendant must prove a lack of due diligence on the part of the plaintiff which resulted in some prejudice to the defendant. The concept of "due diligence" on the part of the plaintiff encompasses the situation in which the plaintiff does not know of the condition giving rise to the claim (in your example, the fence) for a period of time... the question is whether the plaintiff knew or should have known of the condition.