sample_simple_form.rb

= simple_form_for @post, html: { class: 'form-horizontal' }, defaults: { wrapper: :horizontal_form }  do |f|
  = f.error_notification

  .form-inputs
    = f.input :first_name
    = f.input :last_name
    = f.input :email

  .form-actions
    = f.button :submit

simple_form_bootstrap3_horizontal_form_wrapper.rb

# http://stackoverflow.com/questions/14972253/simpleform-default-input-class
# https://github.com/plataformatec/simple_form/issues/316

inputs = %w[
  CollectionSelectInput
  DateTimeInput
  FileInput
  GroupedCollectionSelectInput
  NumericInput
  PasswordInput
  RangeInput
  StringInput
  TextInput
]

inputs.each do |input_type|
  superclass = "SimpleForm::Inputs::#{input_type}".constantize

  new_class = Class.new(superclass) do
    def input_html_classes
      super.push('form-control')
    end
  end

  Object.const_set(input_type, new_class)
end

# Use this setup block to configure all options available in SimpleForm.
SimpleForm.setup do |config|
  config.boolean_style = :nested

  config.wrappers :bootstrap3, tag: 'div', class: 'form-group', error_class: 'has-error',
      defaults: { input_html: { class: 'default_class' } } do |b|

    b.use :html5
    b.use :min_max
    b.use :maxlength
    b.use :placeholder

    b.optional :pattern
    b.optional :readonly

    b.use :label_input
    b.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
    b.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
  end

  config.wrappers :horizontal_form, tag: 'div', class: 'form-group', error_class: 'has-error',
      defaults: { input_html: {} } do |b|

    b.use :html5
    b.use :min_max
    b.use :maxlength
    b.use :placeholder

    b.wrapper tag: 'label', class: 'col-sm-4 control-label' do |label|
      label.use :label_text
    end

    b.wrapper tag: 'div', class: 'col-sm-8' do |input|
      input.use :input
      input.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
    end

    b.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
  end

  # Wrappers for forms and inputs using the Twitter Bootstrap toolkit.
  # Check the Bootstrap docs (http://getbootstrap.com/)
  # to learn about the different styles for forms and inputs,
  # buttons and other elements.
  config.default_wrapper = :bootstrap3
end