...
   public ConfiguredTemplateDefinition() {
        this.templateAvailability = new TemplateAvailability() {

            @Override
            public boolean isAvailable(Node content, TemplateDefinition templateDefinition) {
                // by default a template is available only in the website workspace
                try {
                    return content.getSession().getWorkspace().getName().equals("website") &&
                            StringUtils.substringAfter(templateDefinition.getId(), ":").startsWith("pages/");
                } catch (RepositoryException e) {
                    return false;
                }
            }
        };
    }
    ...