DIY Projects

8×12 Greenhouse Plans

Author profile picture Written by Joseph Truini On April 23, 2024 • 15 min read

In this article, we are going to teach you how to build an 8×12 lean-to greenhouse, perfect for extending your growing season and providing a controlled environment for your plants. Whether you’re a seasoned gardener or just starting out, this step-by-step guide will walk you through the entire process, from choosing the ideal location to installing the polycarbonate panels and finishing touches.

So, let’s get started and build your very own 8×12 lean-to greenhouse!

Unit Qty Size Note
Pressure-treated lumber (2˝x 4˝) pcs 18 12´ Joists, Rim joists, Studs, Bottom plates, Top plates, Cripple studs, Gable wall studs, Horizontal girts, Vertical girts, Ventilation header
Pressure-treated lumber (4˝x 4˝) pcs 7 12´ Studs, Door header
Pressure-treated lumber (2˝x 6˝) pcs 7 10´ Rafters
Concrete cub.ft. 6,98 Foundation
Gravel cub.ft. 8 Flooring
Polycarbonate (0.314˝) pcs 23 4´ x 8´ Sheathing
Screws (1˝) pcs 500
Screws (2˝) pcs 80
Wood screws (3˝) pcs 120
Wood screws (5˝) pcs 260
Screws (5˝) pcs 50
3/8˝ x 7˝ Steel Expansion Anchor pcs 75
Washers pcs 250
H-type Connectors (0.314˝-0.4˝) pcs 4 10´
U-type Edge cups (0.314˝-0.4˝) pcs 5 10´
Hook and Eye (6˝) pcs 4
Hinges pcs 14
Door pull pcs 2
Interior shopping list Unit Qty Size Note
Pressure-treated lumber (2˝x 4˝) pcs 16 Legs, Shelf supports, Joists
Plywood (5/8˝) pcs 2 4´ x 8´ Sheathing
Wood screws (2˝) pcs 100
Wood screws (3˝) pcs 20
Wood screws (5˝) pcs 40
Greenhouse Main Frame Unit Qty Size Note
Concrete cub.ft. 6,98 Concrete posts
Pressure-treated lumber (2˝x 4˝) pcs 2 Rim joists
Pressure-treated lumber (2˝x 4˝) pcs 2 12´ Joists
Pressure-treated lumber (2˝x 4˝) pcs 2 10´-9˝ Studs
Pressure-treated lumber (4˝x 4˝) pcs 4 10´-9˝ Studs
Pressure-treated lumber (2˝x 4˝) pcs 2 3´-6˝ Bottom plates
Pressure-treated lumber (2˝x 4˝) pcs 2 12´ Top plates
Pressure-treated lumber (4˝x 4˝) pcs 1 Door header
Pressure-treated lumber (2˝x 4˝) pcs 1 3´-5˝ Cripple stud
Pressure-treated lumber (4˝x 4˝) pcs 2 6´-9˝ Studs
Pressure-treated lumber (2˝x 4˝) pcs 15 6´-9˝ Studs
Pressure-treated lumber (2˝x 4˝) pcs 1 12´ Bottom plate
Pressure-treated lumber (2˝x 4˝) pcs 2 7´-5˝ Top plate
Pressure-treated lumber (2˝x 4˝) pcs 2 7´-5˝ Bottom plate
Polycarbonate (0.314˝) pcs 2 3´-6˝ x 7´-4˝ Sheathing
Polycarbonate (0.314˝) pcs 2 3´-6˝ x 4´-4 3/4˝ Sheathing
Polycarbonate (0.314˝) pcs 1 8 3/4˝ x 5´ Sheathing
Polycarbonate (0.314˝) pcs 3 4´ x 7´-6 3/4˝ Sheathing
Polycarbonate (0.314˝) pcs 4 4´ x 7´-2˝ Sheathing
Polycarbonate (0.314˝) pcs 2 2´-7 1/4˝ x 4´ Sheathing
Polycarbonate (0.314˝) pcs 2 2´-5 3/4˝ x 4´ Sheathing
Gravel cub.ft. 8 Flooring
Greenhouse Roof Unit Qty Size Note
Pressure-treated lumber (2˝x 6˝) pcs 7 9´-2 1/4˝ Rafters
Pressure-treated lumber (2˝x 4˝) pcs 2 1´-11 1/2˝ Gable wall studs
Pressure-treated lumber (2˝x 4˝) pcs 3 1´-1/4˝ Gable wall studs
Pressure-treated lumber (2˝x 4˝) pcs 2 3´-7 1/2˝ Ventilation header
Polycarbonate (0.314˝) pcs 3 4´ x 8´ Sheathing
Polycarbonate (0.314˝) pcs 2 1´-2 1/2˝ x 6´ Sheathing
Greenhouse Door Unit Qty Size Note
Pressure-treated lumber (2˝x 4˝) pcs 6 2´-2 1/4˝ Horizontal girts
Pressure-treated lumber (2˝x 4˝) pcs 4 7´-1 3/4˝ Vertical girts
Pressure-treated lumber (2˝x 4˝) pcs 4 3´-11 1/2˝ Angled girts
Polycarbonate (0.314˝) pcs 2 2´-5 3/4˝ x 7´-1 3/4˝ Sheathing
Greenhouse Ventilations Unit Qty Size Note
Pressure-treated lumber (2˝x 4˝) pcs 4 2´-4 3/4˝ Horizontal girts
Pressure-treated lumber (2˝x 4˝) pcs 4 3´-1/4˝ Vertical girts
Pressure-treated lumber (2˝x 4˝) pcs 4 3´-3 1/4˝ Horizontal girts
Pressure-treated lumber (2˝x 4˝) pcs 4 1´-11 1/4˝ Vertical girts
Polycarbonate (0.314˝) pcs 2 2´-4 3/4˝ x 3´-3 3/4˝ Sheathing
Polycarbonate (0.314˝) pcs 2 3´-6 3/4˝ x 3´-9 1/2˝ Sheathing
Greenhouse Enterior – Shelves Unit Qty Size Note
Pressure-treated lumber (2˝x 4˝) pcs 8 Legs
Pressure-treated lumber (2˝x 4˝) pcs 8 Joists
Pressure-treated lumber (2˝x 4˝) pcs 8 Joists
Pressure-treated lumber (2˝x 4˝) pcs 12 1´-9˝ Shelf support
Plywood (5/8˝) pcs 4 2´ x 5´-3˝ Sheathing
Fasteners & Hardware Unit Qty Size Note
Screws (1˝) pcs 500
Screws (2˝) pcs 80
Wood screws (2˝) pcs 100
Wood screws (3˝) pcs 140
Wood screws (5˝) pcs 300
Screws (5˝) pcs 50
3/8˝ x 7˝ Steel Expansion Anchor pcs 75
Washers pcs 250
H-type Connectors (0.314˝-0.4˝) ft 31
U-type Edge cups (0.314˝-0.4˝) ft 43
Hook and Eye (6˝) pcs 4
Hinges pcs 14
Door pull pcs 2

Step 1: Choose and Prepare Location for Greenhouse

1.1. The best location to build a greenhouse is on the south side of your property where it’ll be exposed to the most amount of sunshine. If that’s not possible, then locate it on the east or west side of your yard, so it’ll get morning or late-afternoon sun. Wherever you choose to place your greenhouse, make sure the area is not too shady. Here are a few other considerations:

  • Sun exposure: The location should receive plenty of direct sunlight, as plants need light to grow.
  • Easy access: The location should be easily accessible for watering and maintenance.
  • Drainage: The location should have good drainage to prevent water from pooling around the greenhouse.
  • Wind: The location should be protected from strong winds, which can damage the greenhouse or plants.
  • Temperature: Consider the average temperature and climate conditions in the area.
Choosing and Preparing Location for the Greenhouse
Choosing and Preparing Location for the Greenhouse

Step 2: Ground Works

2.1. Clear the area where you want to build the greenhouse and layout for the foundation, using the below illustration as a guide.

2.2. For the foundation, use a power auger to dig ten 8″ diameter holes to a depth of 2′.

Clearing the area and preparing the grounds for the greenhouse installation
Clearing the area and preparing the grounds for the greenhouse installation

Step 3: Foundation Preparation

3.1. Mix up a batch of concrete and pour it into the holes, filling each one flush with the surrounding ground. Let the concrete cure overnight. 

3.2. Once concrete has cured, foundation is ready for the next step.

Preparing foundation for the greenhouse installation
Preparing foundation for the greenhouse installation

Step 4: Foundation Preparation

4.1. Make the foundation base of the greenhouse–called the mudsill–out of pressure-treated 2x4s. Cut two 8′ long 2x4s, and two 12’ long 2x4s; miter-cut both ends of all four boards to 45 degrees, as illustrated below. 

4.2. Use a carpenter’s framing square to confirm that the corners of the mudsill are 90°. Then, fasten the mudsill to the concrete footings with 3/8″ x 4″ steel expansion anchors.

4.3. Dig out the grass from within the mudsill, lay down landscape fabric, then fill the area with gravel. You will need about 8 cu. ft. of gravel.

Preparing foundation for the greenhouse installation
Preparing foundation for the greenhouse installation

Step 5: Assemble the Front Wall

5.1. Build the frame for the front wall out of pressure-treated 2x4s and 4x4s. Use the drawing below as a reference. Cut four 4x4s and two 2x4s to 10′-9″. Two 4x4s will be used as wall studs at the ends of the wall, and the two other 4x4s will form the trimmer studs on either side of the doorway’s rough opening. The two 2x4s will be used as standard wall studs.

5.2. Next, cut two 2x4s to 3′-6″ for the bottom wall plates, one 2×4 to 12′ to serve as the continuous top wall plate. Also cut one 4×4 to 5′ for the door header and a 2×4 to 3′-5″ for the cripple stud to go between the top plate and header.

5.3. Fasten the frame parts together with 5″ screws.

5.4. Using a carpenter’s framing square, check the corners to make sure they are 90°.

Assembling the front wall of the greenhouse
Assembling the front wall of the greenhouse

Step 6: Assemble the Back Wall

6.1. Build the rear wall out of pressure-treated 2x4s and 4x4s, using drawing below as a reference. Cut two 4x4s to 6′-9″ to serve as the end wall studs, and five 2x4s to 6′-9″ for the wall studs. Also one two 2x4s to 12′ to form the top and bottom wall plates.

6.2. Fasten the parts together with 5″ screws.

6.3. Using a carpenter’s framing square, check the corner to make sure they are 90°.

Assembling the back wall of the greenhouse
Assembling the back wall of the greenhouse

Step 7: Assemble the Left Wall

7.1. Build the left wall entirely out of pressure-treated 2x4s, including five wall studs cut to 6′-9″, and the top and bottom wall plates each cut to 7′-5″.

7.2. Attach the parts with 5″ screws.

7.3. Using a carpenter’s framing square, check the corner to make sure they are 90°.

Assembling the left wall of the greenhouse
Assembling the left wall of the greenhouse

Step 8: Assemble the Right Wall

8.1. The right wall is identical to the left wall: Cut five wall studs to 6′-9″, and the top and bottom wall plates to 7′-5″.

8.2. Attach the parts with 5″ screws.

8.3. Using a carpenter’s framing square, check the corner to make sure they are 90°.

Assembly of the right wall of the greenhouse
Assembly of the right wall of the greenhouse

Step 9: Assemble the Roof Frame

9.1. Cut the seven roof rafters out of pressure-treated 2x6s, as shown in the drawing below. Cut each rafter to 9′-2 1/4″. Trim one end of each rafter to sit flush with the top plate on the rear wall. Then, cut a bird’s mouth notch at the opposite end of the rafters, so they sit flat on the top plate of the front wall.

9.2. Fasten the rafters with 5″ screws.

Assembly of the greenhouse's roof frame
Assembly of the greenhouse's roof frame

Step 10: Assemble the Gable Wall Studs

10.1. Cut pressure-treated 2x4s to form the studs in the upper section of the left and right gable-end walls. Using drawing below as a reference, cut two 2×4 studs to 1′-11 1/2″ and two 1′-1/4″. Then cut two 2x4s to 3′-7 1/2″ to frame the rough opening for an air vent.

10.2. Fasten the gable wall studs with 5″ screws.

10.3. Using a carpenter’s framing square, check the corner to make sure they are 90°.

Assembly of the Greenhouse's Gable Wall Studs
Assembly of the Greenhouse's Gable Wall Studs

Step 11: Install the Polycarbonate for the Front Wall

11.1. Cut 5/16″ polycarbonate panels into five pieces for the front wall, using the drawing below as a reference. Cut two pieces to 3′-6″ x 7′-4″, two to 3′-6″ x 4′-4 3/4″, and one to 8 3/4″ x 5′ sheet.

11.2. Secure the polycarbonate with 1″ screws and washers.

Installation of the Polycarbonate for the Front Wall of the Greenhouse
Installation of the Polycarbonate for the Front Wall of the Greenhouse

Step 12: Install the Polycarbonate for the Back Wall

12.1. Cut 5/16″ polycarbonate for the back wall, using the drawing below as a reference. Cut three pieces to 4′ x 7′-6 3/4″.

12.2. Secure the polycarbonate with 1″ screws and washers.

Installation of the Polycarbonate for the Back Wall of the Greenhouse
Installation of the Polycarbonate for the Back Wall of the Greenhouse

Step 13: Install the Polycarbonate for the Left Wall

13.1. Cut 5/16″ polycarbonate for the left wall, using drawing below as a reference. Cut two pieces to 4′ x 7′-2″, one to 2′-7 1/4″ x 4′, and one to 2′-5 3/4″ x 4′ sheet.

13.2. Secure the polycarbonate with 1″ screws and washers.

Installation of the Polycarbonate for the Left Wall of the Greenhouse
Installation of the Polycarbonate for the Left Wall of the Greenhouse

Step 14: Install the Polycarbonate for the Right Wall

14.1. Cut 5/16″ polycarbonate for the left wall, using drawing below as a reference. Note that these four parts are identical to the parts cut for the left wall.

14.2. Cut two pieces to 4′ x 7′-2″, one to 2′-7 1/4″ x 4′, and one to 2′-5 3/4″ x 4′ sheet.

14.3. Secure the polycarbonate with 1″ screws and washers.

Installation of the Polycarbonate for the Right Wall of the Greenhouse
Installation of the Polycarbonate for the Right Wall of the Greenhouse

Step 15: Install the Polycarbonate for the Roof

15.1. Cut 5/16″ polycarbonate for the roof sheathing, using the drawing below as a guide. You will need three 4′ x 8′ panels and two pieces cut to 1′-2 1/2″ x 6′.

15.2. Use H-type connectors (5/16”) and U-type edge cups (5/16″) to ensure a waterproof roof. You will need 31 linear ft. of H-type connectors and 43 linear ft. of U-type edge cups. Be sure to apply silicone sealant to the joints when installing the connectors and cups to enhance the water tightness of the roof. 

15.3. Secure the polycarbonate with 1″ screws and washers.

Installation of the Polycarbonate for the Greenhouse's Roof
Installation of the Polycarbonate for the Greenhouse's Roof

Step 16: Assemble and Install Greenhouse Doors

16.1. Build the two hinged greenhouse doors out of pressure-treated 2x4s; join the parts with 5″ screws. Cut six 2x4s to 2′-2 1/4″ for the horizontal rails, four 2x4s to 7′-1 3/4″ for the vertical stiles, and four 2x4s to 3′-11 1/2″ to serve as diagonal braces. 

16.2. Cut two 5/16″ polycarbonate sheets to 2′-5 3/4″x 7′-1 3/4″ to cover the doors.

16.3. Secure the polycarbonate with 1″ screws and washers.

16.4. Attach each door to the front wall with two hinges. Also install a handle to each door.

Assembling and Installation of the Greenhouse's Doors
Assembling and Installation of the Greenhouse's Doors

Step 17: Assemble and Install Greenhouse Ventilations

17.1. Next, make the four air vents for the greenhouse. There are two vents above the doorway and one on each end wall of the greenhouse. The vents are necessary to prevent the interior of the greenhouse from getting too hot. 

17.2. As shown in Node A (page 28), build the rectangular frame for the vents that go above the doorway out of pressure-treated 2x4s. Cut four 2x4s to 2′-4 3/4″ for the horizontal top and bottom of the frame, and four boards cut to 3′-1/4″ that will be vertical slides of the frame. Fasten the frame together with 3 in. screws.

17.3. Cut two pieces of 5/16″ polycarbonate to 2′-4 3/4″ x 3′-3 3/4″ and secure them to the vent frames with 1 in. screws and washers. 

17.4. According to the Node B (page 28), build the ventilation frame for each side wall using pressure-treated 2x4s. Cut four 2x4s to 3′-3 1/4″ and four to 1′-11 1/4″. Fasten the frame together with 3 in. screws.

17.5. Cut two pieces of 5/16″ polycarbonate to 1′-11 1/4″ x 3′-6 3/4″, then secure them to the vent frames with 1″ screws and washers.
17.6. Fasten the vents to the greenhouse with hinges and 1’ screws. Then, according to the Node C (page 29), install a hook-and-eye closure to each vent.

Assembling and Installing the Greenhouse's Ventilations
Assembling and Installing the Greenhouse's Ventilations
Assembling and Installing the Greenhouse's Ventilations

Step 18: Assemble and Install Greenhouse Interior

18.1. Build two potting benches for the interior of the greenhouse. The benches provide a convenient work surface for potting and pruning plants, and below the work surface is a handy storage shelf. Here are detailed step-by-step instructions for building the benches:

Assembly and Installation of the Greenhouse Interior
Assembly and Installation of the Greenhouse Interior

Step 19: Assemble Main Frame

19.1. Build the structural frames of the two potting benches out of pressure-treated 2x4s. Cut the eight 2×4 legs to 4′, and then cut eight 2x4s to 2′ for the horizontal end rails, and eight 2x4s to 5′ for the horizontal rails along the front and rear of the frame. 

19.2. Fasten the frame parts together with 3″ screws.
19.3. Using a carpenter’s framing square, check the corner to make sure they are 90°.

Assembling of the main frame of the Greenhouse's interior
Assembling of the main frame of the Greenhouse's interior

Step 20: Assemble the Shelves

20.1. Cut short lengths of pressure-treated 2x4s to span between the front and rear rails, as shown below. Cut twelve 2x4s to 1′-9” and secure them with 3″ screws.

20.2. Using a carpenter’s framing square, check the corner to make sure they are 90°.

Assembly of the shelves of the Greenhouse's interior
Assembly of the shelves of the Greenhouse's interior

Step 21: Install Shelves with Plywood

21.1. Cover the work surface and storage shelf with 5/8″ plywood. Cut four plywood pieces to 2′ x 5′-3″.

21.2. Fasten plywood to the 2×4 frame with 2″ screws.

21.3. Apply two coats of exterior-grade marine varnish to plywood.

Plywood application to the shelves of the Greenhouse
Plywood application to the shelves of the Greenhouse