anyone???

Basically what Magnus said.

Create the page, then disallow the creation of it. You could also do it with permissions, but that is more of a hassle I guess.

Thanks guys was looking for a more elegant solution but sometimes the easy root will do it :-)

This answer is a bit late but there's a possibility that someone may find this useful.

1. Make sure the root page is the only page that allows that type as a child

2. Use a ContentService notification such that if an attempt is made to save a second node with that doc type then cancel that operation.

Example below for a doc type with alias "subSettings"

public class RestrictPageType : INotificationHandler<ContentSavingNotification>
{
    private readonly IContentService _contentService;
    private readonly IContentTypeService _contentTypeService;

    public RestrictPageType(IContentService contentService, IContentTypeService contentTypeService)
    {
        _contentService = contentService;
        _contentTypeService = contentTypeService;
    }

    public void Handle(ContentSavingNotification notification)
    {
        string thereCanBeOnlyOne = "subSettings";
        IContentType subSettingType = _contentTypeService.Get(thereCanBeOnlyOne) ?? throw new Exception($"Invalid alias: {thereCanBeOnlyOne}");
        foreach (var node in notification.SavedEntities)
        {
            if (node.ContentType.Alias.Equals(thereCanBeOnlyOne))
            {
                IContent? publishedPage = _contentService
                     .GetPagedOfType(subSettingType.Id, 0, 9999, out long _, null)
                     .FirstOrDefault();
                if (publishedPage != null && publishedPage.Id != node.Id)
                {
                    notification.CancelOperation(new EventMessage($"There can be only one {thereCanBeOnlyOne}",
                    $"Only one page of type {subSettingType.Name} can be in the site",
                    EventMessageType.Error));
                }
            }
        }
    }
}

public class RestrictPageTypeComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
    {
        builder.AddNotificationHandler<ContentSavingNotification, RestrictPageType>();
    }
}

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