Where a planning enforcement notice has been served and not complied with it is not a defence to challenge the validity of the enforcement notice at proceedings brought for non-compliance. There is a large body of case law to support this proposition and it is settled law. The arena for challenge of the notice itself is the notice appeals procedure. The position is slightly different where a notice is clearly bad on its face or is clearly invalid. In these circumstances it can be argued that it is an abuse of the process of the Court for a prosecution to continue in such circumstances. Of course it is impossible to lay down a general principle as far as this is concerned as the application will be fact specific.
The most obvious defence to breach of a planning enforcement notice is available under s179(3) of the Town and Country Planning Act 1990 which reads as follows:
(3) In proceedings against any person for an offence under subsection (2), it shall be a defence for him to show that he did everything he could be expected to do to secure compliance with the notice.
This is a statutory defence which allows a defendant to be acquitted if they do all that could be expected to comply with the notice; the application of this defence deserves some examination.
The burden of proving the statutory defence rests upon the defendant; the standard is to the ‘balance of probability’- more likely than not. The requirement of ‘reasonableness’ applies an objective test to what the defendant can be expected to have done.
The test is relatively strictly interpreted, the Court of Appeal in R. v. Revitt and others  stated that in examining any such defence raised a court should:
“…be fairly rigorous in the proof which it expects or demands of a defendant, and not allow itself to be hoodwinked by protestations of impecuniosity on behalf of any individual”
A review of the case law in respect of this defence was undertaken in the case of Sevenoaks District Council v. Harber  EWHC 708. From that, the following propositions can be discerned:
a. Personal circumstances of a defendant can be taken into account if they mean that the defendant was incapable of complying with the notice;
b. Before the statutory defence can arise the owner must show that compliance with the notice is not within their powers;
c. The relevant question is whether the defendant is incapable of compliance, not whether there is a reasonable excuse for noncompliance and the defence is concerned only with the ability to comply, not wider issues such as hardship or reasonableness of compliance.
The law is of course open to interpretation; a very useful case for the defence is R v Warwick County Court ex p. White . In that case the Court was clear in its view that the availability of the defence under s179(3) was dependant upon the question of the control of the defendant over the property. As Lord Bingham stated:
The meaning of section 179 is clear and unambiguous. Where it is within the power of the owner of the land to comply with the notice without the assistance of others, no question of a defence under subsection (3) arises. Before a defence can arise under that subsection, the owner must show that compliance with the notice is not within his own unaided powers, otherwise no question of his having to secure compliance with the notice can arise....We accept, as does counsel for the prosecution, that the phrase 'everything he could be expected to do' must implicitly be read as 'reasonably expected'. It applies an objective criterion of reasonableness, having regard to all the relevant circumstances, in particular any disabilities to which the owner of the land is subject.
It is often said that failure to comply with a planning enforcement notice is an offence of ‘strict liability’. This is due to the fact that knowledge is not needed to prove the offence. Whilst this is true, there is a defence of lack of knowledge of the service of a copy of the notice contained under subsection 7 as outlined below:
(7) Where— (a) a person charged with an offence under this section has not been served with a copy of the enforcement notice; and (b) the notice is not contained in the appropriate register kept under section 188,
it shall be a defence for him to show that he was not aware of the existence of the notice.
It is therefore essential that legal representatives check the status of the 188 register as this can be crucial in establishing a defence under this section.
There is also a defence available of ‘lawful use’. What is crucial in these circumstances is the extent to which an alleged failure to comply with a valid notice consists of the carrying on of an activity which the defendant is otherwise entitled to carry on without further planning permission, and to construe the notice as not extending to that activity. Thus if the breach to which the notice is directed is the growing of vegetables on a commercial basis from a greenhouse, a requirement simply to cease using the greenhouse for the growing of vegetables may be construed as not extending to the growing of vegetables for consumption at home, because that is an activity ancillary to the primary use of the planning unit. This approach was confirmed by the Court of Appeal in R v Harfield .